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R134a was recognized as probably one of the most important refrigerants of the two past decades. Among the proposed alternative fluids, there are certainly isobutane (R600a) and R152a. This article presents about 200 new heat transfer coefficient and pressure drop data obtained during flow boiling of R152a and R600a inside a smooth copper tube having an internal diameter of 4 mm. Three saturation temperatures were considered for each refrigerant, from 5 °C to 20 °C. Furthermore, for each temperature studied, the heat flux was varied between 15 and 30 kW m−2 and the refrigerant mass flux from 100 to 400 kg m−2 s−1. After presenting the new data, a critical comparison was proposed between the performance of these refrigerants and R134a. Finally, some classic correlations available in the literature have been implemented. The deviations between the calculated and experimental values were reported and commented. [ABSTRACT FROM AUTHOR]

This study investigates the flow boiling heat transfer characteristics of refrigerants R134a and R407c in horizontal micro-fin tubes. We conducted experiments across a range of heat flux (15–35 kW·m−2), mass flux (50–250 kg·m−2 s−1), and saturation temperature (15 °C-25°C) conditions. Our findings reveal that R134a consistently exhibits a higher heat transfer coefficient (HTC) compared to R407c, making it advantageous for applications requiring efficient heat transfer. However, R134a also shows a tendency towards premature dry-out flow patterns at higher heat fluxes, which could limit its effectiveness under certain conditions. In contrast, R407c demonstrates more stable flow regimes, maintaining wavy-annular and annular flows without early dry-out, which highlights its robustness at higher heat fluxes. Flow pattern maps indicate that higher heat fluxes promote the formation of wavy and annular flow patterns in both refrigerants, with R134a transitioning to dry-out flows more readily than R407c. The influence of saturation temperature on HTC was also significant, with lower temperatures resulting in higher HTCs for both refrigerants. This can be attributed to the increased thermodynamic driving force for phase change at lower temperatures. Our study aligns with previous research, corroborating the critical role of micro-fin tubes in enhancing HTC. The findings have practical implications for the design and optimization of refrigeration systems, emphasizing the need to carefully select refrigerants and operating conditions to achieve efficient and stable heat transfer performance. [ABSTRACT FROM AUTHOR]

In the present study, an experimental investigation is conducted to study the nucleate boiling characteristics on a smooth tube having a low-GWP azeotropic refrigerant, R513A. The heat flux ranges from 5 to 90 kW/m2, and saturation temperatures are 0, 6, 10, 16, and 22 °C, respectively. The performance of R513A is compared against that of R134a. The performance is in terms of heat transfer coefficient (HTC). HTCs normally increased with the rise of heat flux and saturation temperature. However, the HTCs of R513A exceed those of R134a by approximately 20–25 %. Additionally, comparative analysis of the test data and some existing correlations are made. The compared correlations include three correlations based on reduced pressure (Cooper, Ribatski and Jabardo, and Jung), as well as two correlations based on thermophysical properties. It is found that the Jung correlation gives the best overall predictive ability against the R513A data with a deviation ranging from 5 % to 10 %. [ABSTRACT FROM AUTHOR]

Aiming at the heat dissipation of electronic components with high heat flux, five phase change heat transfer cold plates with different structures and processing methods were designed, the effects of cold plate structural parameters, heat flux, refrigerant flow rate, and heat source layout on heat transfer performance, resistance of cold plate, and the pump power consumption were experimentally studied. The results show that the designed perforated cold plate has excellent heat transfer performance. The designed perforated cold plate maintains a temperature difference of less than 25 ℃ between the heat source surface and the refrigerant inside the cold plate when dissipating a total heat output of 5 000 W and a heat flux of 38 W/cm~2, with cold plate resistance less than 30 k Pa and pump power consumption less than 35 W. The processing method of cold plate has a significant influence on its heat transfer effect. The cold plate should adopt integrated design as far as possible to reduce the link of heat transfer and avoid the contact thermal resistance caused by the chimerism of different materials. When multiple heat sources have different heat flux densities, placing the higher heat flux heat source closer to the inlet of the cold plate achieves better heat transfer efficiency. [ABSTRACT FROM AUTHOR]

• Establishment and verification of a rig for refrigerants' pool boiling. • Heat transfer coefficients were obtained for the R134a pool boiling on 6 surfaces with various roughness and wettability under 3 saturation temperatures. • A phenomenon of bubble behavior above the boiling surface was spotted and observed with a high-speed camera. In order to investigate pool boiling heat transfer characteristics of refrigerants on different surfaces, an experimental rig is established and verified in this study. The pool boiling heat transfer experiment is conducted with R134a at three saturation temperatures. Six surfaces with Ra roughness ranging from 0.133 to 5.742 μm and two types of wettability are tested in the study. The heat flux varies from 40 to 280 kW/m2. Experimental results indicate that higher roughness is favorable to boiling heat transfer. Shieldex coating on copper surfaces can deteriorate heat transfer by 20% approximately. Visualization analysis was carried out with a high-speed camera to study bubble behavior. A new-found mode of bubble departing was observed at lower heat flux. Besides, the data were correlated in the form of Rohsenow correlation, and comparison and supplement are concluded based on data obtained in this study. [ABSTRACT FROM AUTHOR]

• A quantification method is proposed to calculate the flow rate distribution of microchannel heat exchangers. • Liquid and vapor mass flow rates are calculated with the method, and it shows how refrigerant is distributed in the inlet header. • The potential of improving the heat capacity of the evaporator with uniform distribution is calculated and discussed. Maldistribution of fluid among the ports reduces the heat capacity of a microchannel heat exchanger. It is essential to understand the maldistribution mechanism to optimize the design of the heat exchanger. Quantification of the distribution helps analyze the maldistribution issue. This study proposes a method to quantify the flow distribution and capacity potential in the microchannel heat exchangers by integrating a microchannel heat exchanger model and infrared thermography. The method calculates the liquid and vapor mass flow rate at the inlet of each microchannel tube by comparing the wall temperature of a heat exchanger from simulation and the infrared image. The method is validated with tests of a 49-tube horizontal microchannel evaporator with R134a. The exit region of the top tubes has an extremely high surface temperature, which forms a superheat region. More liquid flows to the tubes located at the center of the heat exchanger. The total mass flow rate is low when the tube has a long superheat region. The heat capacity of the tested evaporator has 9.6–21.3% improvement potential when uniformly distributed. When the refrigerant outlet superheats of the evaporator is lower, the improvement potential is higher. In the superheat region, the heat flux is much lower than the average value. This quantification method provides a tool for a more profound investigation of the maldistribution issue of a microchannel evaporator, and it can be expanded to heat exchangers with different structures. [ABSTRACT FROM AUTHOR]

• Heat transfer of supercritical R1234yf was experimentally investigated in a horizontal micro-fin tube. • Operating parameter effects on heat transfer were analyzed. • Heat transfer comparisons were conducted between R1234yf and R134a. • Prediction accuracy of existing correlations for R1234yf was analyzed. R1234yf is regarded as an ideal substitute for R134a in supercritical organic Rankine cycles. This study experimentally analyzed the convective heat transfer coefficients of supercritical R1234yf in a micro-fin tube. The experiments show the influence of the system operating parameters including pressure, heat flux and mass flux on the heat transfer. Then, this paper compares the heat transfer rates for R1234yf and R134a at supercritical pressures and the ability of existing correlations to predict the heat transfer coefficients with R1234yf. The results show that as q / G increases, the buoyancy increases Nu bottom , while the variations in Nu top are divided into two regions based on the bulk fluid enthalpy. The influence of pressure on the heat transfer is also related to the bulk fluid enthalpy. When the bulk fluid enthalpy is less than a critical value, Nu bottom and Nu top both increase with decreasing pressure and then decrease above this critical enthalpy. The heat transfer coefficient is no longer enhanced at the top with large buoyancy forces. For low mass fluxes, the heat transfer coefficients of R134a and R1234yf are similar. For high mass fluxes, the heat transfer coefficients of R134a are higher than those of R1234yf. The Wang correlation, that was based on R134a data, more accurately predicts the heat transfer coefficients than other correlations for supercritical R1234yf in the horizontal micro-fin tube. Among all the 4050 experimental points, 90.17% of Nu top and 84.49% of Nu bottom were predicted with errors of less than 30% by the Wang correlation. [ABSTRACT FROM AUTHOR]

• The ability of CFD codes to predict the complex phenomena of subcooled flow and the Critical Heat Flux (CHF). • The implementation of wall mechanistic models in the CFD codes predicting boiling behavior. • Verification and validation of CFD codes concerning NPP design. • Fluid-to-fluid similarity methods have been used to select the potential surrogate fluid. • The CFD results have been compared against experiment results for vertical and horizontal subcooled boiling flow. In this study, we present the possibility of substituting the primary fluid Freon 11 (CCl 3 F, R11) used in the experimental thermohydraulic test facility which offers the advantage to obtain phase change at relatively low heat fluxes and pressure if compared to water working fluid. Nevertheless, R11 is a CFC (Chlorofluorocarbures) part of the gases that contribute to the degradation of the ozone layer. For this purpose, four working fluids (non-CFC) are judiciously selected to replace the R11, namely the R123, R134a, R143a and CO2, and this through multidimensional Computational Fluid Dynamics (CFD) modeling of steady state flow boiling phenomenon. The calculation model covers a wide range of mass fluxes (435 – 1443 kg/m2s) and heat fluxes (1.2– 36 W/cm2); it predicts the heat transfer rates and transitions points for each flow regime. In this research, the experiment's data were compared to the outcomes of Ansys-CFX computations, using a steady Reynolds-Averaged Navier-Stokes (Steady-RANS) model, to investigate the best replacement refrigerant for R11. The turbulence effects on the mean flow and near wall are taken into consideration using the Shear Stress Transport (SST k-ω) turbulence model. The results from the four fluids were transposed to their water equivalent and compared to those deduced from the experimentation and correlations available in the literature to select the appropriate substitution fluid. The results qualitative and quantitative for vertical and horizontal subcooled boiling flow at the CHF location show that, the most effective surrogate fluid is R123 (HCFC), is still widely used in the refrigeration and air-conditioning industry, then R134a (HFC), then R143a (HFC). [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the film condensation heat transfer characteristics of a horizontal smooth tube and an enhanced tube using R134a, R1233zd(E) and R1234ze(E). The objectives of this paper are to develop experimental correlations of heat transfer for enhanced tubes used in a film type condenser and to provide a guideline for design of the film type condenser. Tests are performed on a horizontally smooth tube with a 19.05 mm diameter and three enhanced tubes. The temperature of the saturated vapor inside the condenser is 38 °C. In the case of the horizontal smooth tube, the film condensation heat transfer coefficients of R1234ze(E) and R1233zd(E) were approximately 10.97% and 10.04% lower than those of R134a, respectively. The experimental values obtained from the horizontal smooth tube agreed well with the theoretical value of Nusselt's film which were within the ±10% error range. In the enhanced tubes experiment, the film condensation heat transfer characteristics according to the number of knurling and Fin per inch were examined. The enhanced tubes E.T(C)-1 and E.T(C)-3 exhibit a fin per inch number value of 55 and 85 and 107, respectively. Moreover, the fin per inch and knurling number of E.T(C)-5 are 60 and 117, respectively. As a result, the film condensation heat transfer coefficient was found to decrease with increasing heat flux, and E.T (C) -5 showed the highest average film condensation heat transfer coefficient for all refrigerants. Based on the experimental results, the effect of the fin per inch number on the heat transfer coefficient outside the tube is greater than that of the knurling number. In this study, the equations for film condensation heat transfer coefficient for each refrigerant and each enhanced were derived tube according to the heat flux, and a total of nine correlations were observed to have an accuracy of ±10% within the experimental range. [ABSTRACT FROM AUTHOR]

The heat transfer characteristics of a thin porous copper-coated wickless heat pipe using R134a as a working fluid is investigated and is compared for its performance with uncoated wickless heat pipe using the same working fluid. An electroplating process was utilised to form a porous structure of copper over the inner surface of the wickless heat pipe. The experiments were carried out in the heat input range between 50 and 250 W. The thermal resistance of heat pipe at three different inclination angles such as 0°, 45° and 90° with horizontal are investigated. The results showed that 45° inclination has the lowest resistance with significant improvement in heat transfer characteristics. The coated wickless heat pipe exhibited a low thermal resistance when compared to uncoated wickless heat pipe. The condenser and evaporator heat transfer coefficients of a coated wickless heat pipe were found to be higher by about 11% and 25%, respectively, when compared to uncoated heat pipe for a heat flux of 10 kW m−2 and inclination of 45°. The magnitudes of dimensionless numbers (such as, Bo, We, Ku and Co) on coated and uncoated wickless heat pipes are also found. [ABSTRACT FROM AUTHOR]

• R1234yf and R1234ze(E) are gaged as R134a low GWP alternatives. • New R1234yf experimental data are presented. • R1234yf, R1234ze(E) and R134a performance is compared. • Several literature correlations are assessed. R134a is ongoing a phase-down process in several countries and R1234yf can be a viable low GWP alternative together with R1234ze(E). Since new experimental data are always requested by researchers and industry to increase the global database, to assess the existing correlations, and to expand the knowledge about the refrigerant applications, in this paper more than 100 new heat transfer coefficient and frictional pressure drop data are presented during R1234yf flow boiling inside a 4 mm inner diameter smooth copper tube. Three saturation temperatures 10°C, 15°C and 20°C were investigated and critically discussed, for each temperature the heat flux ranged from 15 to 30 kW m−2 and the mass velocity from 300 to 600 kg m−2 s−1. Furthermore, some literature correlations were implemented and carefully examined and their deviations are reported. Finally, the R1234yf heat transfer performance is compared against R1234ze(E) and R134a, highlighting the peculiarities of each alternative molecule. R1234ze(E) outperforms the other refrigerants at high vapor qualities but both R1234yf and R1234ze(E) can be viable low GWP substitutes to R134a, if the new design is able to take advantage of the qualities of each refrigerant. [ABSTRACT FROM AUTHOR]

Highlights • Finding condensation heat transfer coefficient for smooth and grooved flat tubes using R134a. • Investigating the effect of mass flux, saturation temperature and heat flux on heat transfer coefficient. • Comparison of heat transfer coefficient data with well-known correlations. • Improving Aker et al. heat transfer correlation [21] for the current experimental data. Abstract Condensation heat transfer coefficient of R134a was obtained in multi-port micro channel smooth and grooved flat tubes of automotive heat exchanger. The test consisted of two smooth and one grooved tube. The test range covered mass flux 490–1600 kg/m2 s, heat flux 5.5–19 kW/m2 and saturation temperature 51–68 °C. Results revealed that condensation heat transfer coefficient increased with an increment of mass flux, vapor quality and decreased with the increase of saturation temperature. The effect of heat flux on heat transfer coefficient was negligible. The grooved tube showed larger heat transfer coefficient than that of smooth tubes. A comparison of the present data of flat tube with existing correlations revealed that the heat transfer coefficient was not reasonably predicted, therefore new correlation was developed for heat transfer coefficient that predicted 86% data within ±30% with a mean error of 16%. [ABSTRACT FROM AUTHOR]

Abstract The heat transfer of supercritical R134a was investigated experimentally in a horizontal micro-fin tube for conditions related to the supercritical heater in trans -critical ORC systems, including the effects of heat flux, pressure, and mass flux. The results showed that Nu top was smaller than Nu bottom due to the buoyancy. The effects of pressure and heat flux on Nu were found to differ in different bulk enthalpy regions and tube positions. Large mass fluxes increase Nu while buoyancy has a stronger effect at low mass fluxes. Previous correlations were evaluated with new correlations developed for Nu in a micro-fin tube. The Bishop correlation gave reasonable predictions of Nu top , but none of the correlations could accurately predict Nu bottom. A pair of more accurate correlations were developed for the top and bottom Nusselt numbers using the dimensionless number π c to account for the effects of the drastic property variations. The new correlations have a mean deviation of 10.3% for Nu top, and 17.8% for Nu bottom, with 96.1% of the experimental data for Nu top and 83.8% of the data for Nu bottom well predicted with deviations smaller than 30%. Highlights • The heat transfer of supercritical R134a was experimentally investigated in a horizontal micro-fin tube. • The effects of pressures and heat fluxes on Nu differs in different bulk regions and tube positions. • Previous correlations were evaluated. • New correlations with better prediction accuracy were provided. [ABSTRACT FROM AUTHOR]

Highlights • Finding frictional pressure drop in multi-port microchannel smooth and grooved tubes using R134a. • Analyzing the effect of mass flux, saturation temperature and heat flux at different vapor quality. • Comparing heat transfer coefficient and pressure drop between Tube X, Tube Y and Tube Z. • Comparing the current experimental frictional pressure drop data with well-known correlations. Abstract Condensation frictional pressure drop was obtained in multiport micro channel smooth and grooved flat tubes using R134a. The study consists of two smooth and one grooved tube. Refrigerant R134a is studied over a mass flux 490–1600 kg/m2s, heat flux 5.5–19 kW/m2 and saturation temperature 51–68 °C. Results reveal that frictional pressure drop increases with an increment of mass flux, vapor quality and decreases with the increase of saturation temperature. The effect of heat flux on frictional pressure drop was negligible. Grooved tube shows the lowest pressure drop and surprisingly highest heat transfer coefficient. The frictional pressure drop data was compared with famous correlations. Zhang and Webb predicted 86% of the data with in ±30%. [ABSTRACT FROM AUTHOR]

Abstract In order to restrict the adverse effects caused by the heat transfer deterioration of the supercritical fluid and to build a cost-effective Trans -critical Organic Ranke Cycle (TORC) system, the heat transfer of supercritical R134a was experimentally investigated in a horizontal ribbed tube for the first time. Knowledge that are important for the design of the supercritical heater and TORC system were explored in detail, including the general features of the heat transfer and effects of heat fluxes, pressures, and mass fluxes. It was found that large heat flux will decrease Nu in most cases, and the effects of pressure differed largely in different working conditions. The working conditions with low heat flux, large mass flue and pressure close to the critical pressure were recommended for the design of the supercritical heater and TORC system. Compared with vertical smooth tubes, the threshold for heat transfer deterioration criterion, Bo , was enlarged at least 7.5 times and the heat transfer was improved around 1.6 times among all the experimental data. Then, different types of previous correlations were evaluated and none of them showed a satisfactory accuracy. Finally, new correlations with better prediction accuracy were built to provide references for the TORC system design. Highlights • The heat transfer of supercritical R134a was experimentally investigated in a horizontal ribbed tube. • The working conditions that benefit a cost-effective TORC were recommended. • The heat transfer in the internally ribbed tube was improved around 1.6 times compared with the smooth tube. • New correlations with better prediction accuracy were provided. [ABSTRACT FROM AUTHOR]

Highlights • Experiments of supercritical R134a flowing in a horizontal tube were conducted. • Effects of mass flux, heat flux and pressure were analyzed in details. • c p affects heat transfer at low q / G , while buoyancy effect dominates at high q / G. • New heat transfer correlations were developed. Abstract This study experimentally investigates the heat transfer characteristics of R134a at supercritical pressures in a 10.3 mm horizontal tube to provide basic heat transfer data and heat transfer correlation. The experiments cover wide parameter ranges of p = 1.02–1.2 p c , G = 400–1500 kg/m2 s, and q ″ = 20–100 kW/m2. The influences of the heat flux, the mass flux and the pressure on the wall temperature and heat transfer coefficient are analyzed. The results show that in a horizontal tube, the buoyancy effect due to density variations causes non-uniformity of the wall temperatures in the circumferential direction. The heat transfer deteriorates along the top surface at high q ″/ G (heat flux to mass flux ratio), while the heat transfer along the bottom surface is enhanced at all operating conditions. The pressure mainly affects the heat transfer coefficient on the bottom surface at small q ″/ G because of the c p variation with pressure. Nine heat transfer correlations are evaluated with the results showing that the Dittus–Boelter type correlations with property modifications have acceptable accuracy for the bottom surface but all of the correlations fail to predict the heat transfer coefficients on the top surface. A new correlation using a buoyancy parameter to include the buoyancy effect is then developed to predict the top surface temperatures and a DB type correlation with property modifications is used to predict the bottom surface temperatures. Results show that the new correlations agree well with the data. [ABSTRACT FROM AUTHOR]

Cryogen spray cooling (CSC) is commonly applied in laser dermatology to protect the epidermis from thermal damage. Many efforts have attempted to improve the cooling capacity of CSC, among which the use of expansion-chambered nozzles is an effectively simple method with considerable potential. This study examined the influences of the expansion-chambered nozzle structure, including the ratios of inlet nozzle diameter to discharge nozzle diameter and of chamber diameter to discharge nozzle diameter on R134a and R404A spray cooling. Fifteen transparent expansion-chambered nozzles with the expansion chamber aspect ratio of 1.0, chamber diameter to discharge nozzle diameter ratios of 5.0–10.0, and inlet nozzle diameter to discharge nozzle diameter ratios of 0.6–1.4 were tested. The internal flow pattern inside the expansion chamber, external spray pattern, and surface heat transfer characteristics of cryogen spray using different nozzles, including the straight-tube nozzle, were investigated. The structure of the expansion chamber was found to have an important effect on the spray patterns and cooling characteristics. The spray radius obviously decreased when the expansion-chambered nozzles were used, and the spray pattern became narrower as the ratio of chamber diameter to discharge nozzle diameter increased. By contrast, the increase in ratio of two nozzle diameters enlarged the spray radius. Surface temperature and heat flux with different nozzles and cryogens showed a similarity, and correlations of surface temperature and heat flux were proposed. The introduction of expansion-chambered nozzles could effectively improve the spray cooling capacity. The minimum average surface temperature during the fully developed spray period could be reached for both R134a spray and R404A spray by an expansion-chambered nozzle with a chamber diameter to discharge nozzle diameter ratio of roughly 5.0 and an inlet nozzle diameter to discharge nozzle diameter ratio of roughly 0.6. [ABSTRACT FROM AUTHOR]

The article presents detailed two-phase adiabatic pressure drops data for refrigerant R134a. Study cases have been set for a mass flux varying from 200 to 400 kg/m2s, at the saturation temperature of 19.4 °C. Obtained experimental data was compared with the available correlations from the literature for the frictional pressure drop during adiabatic flow. Influence of mixture preparation on pressure drop was investigated, for varying inlet subcooling temperature in the heated section. The flow patterns have also been obtained by means of a high-speed camera placed in the visualization section and compared with literature observations. [ABSTRACT FROM AUTHOR]

The paper presents flow characterization of condensation based on experiments with R134a in 6.1 mm inner diameter horizontal round tube. The paper presents flow visualizations and liquid film thickness measurements with mass fluxes from 50 to 200 kg m −2 s −1 and heat fluxes from 5 to 15 kW m −2 , showing the effect of mass flux and heat flux on the onset of condensation, flow regime, film distribution and void fraction. All the measurements are taken at constant pressure of 1.319 MPa, which corresponds to a saturation temperature of 50 °C. The result of flow visualization reveals that the condensation always starts in the bulk superheated region, which is generally defined as the beginning of condensing superheated region, as annular flow and the flow regime is strongly affected by the mass flux, instead of the heat flux. Based on the flow visualization results and underlying physics in the condensing superheated region, a new diabatic flow regime map is proposed to better represent the physics and predict flow regimes in both the two-phase and the condensing superheated region. The range of prediction of the new flow regime map extends over quality one up to the onset of condensation and the early stages of condensation are strictly in annular flow regime. A film thickness measurement technique for round tube is described as well as results of calibration. The film thickness measurement demonstrates that void fraction drops below one in superheated region, which is not taken into account for most conventional void fraction models. In addition, both the mass flux and heat flux affect the void fraction by altering the onset of condensation, a new void fraction model is proposed to include this mechanism. Moreover, having information of film distribution provides an opportunity to more realistically model the film geometry inside of the tube. [ABSTRACT FROM AUTHOR]

Critical heat flux (CHF) experiments were conducted in a uniformly heated horizontal helically-coiled tube to investigate the CHF characteristics at near the critical pressures with R134a as the working fluid. The experiments were performed over a wide range of parameters: pressure from 2.85 to 4.03 MPa, i.e., reduced pressures ( P r) ranging from 0.70 to 0.99, mass flux from 250 to 2100 kg m −2 s −1 , inlet subcooling from 45 to 160 kJ kg −1 and heat flux from 20 to 450 kW m −2 . Two types of CHF phenomenon which are dry-out (DO) and departure from nucleate boiling (DNB) have occurred under the experimental conditions. The DO happens at the lower pressures while the DNB happens at the higher pressures. The wall temperature shows a very different characteristic for the two types of CHF. Under the DO conditions, wall temperature gets a sudden rise firstly at the inner-side of the outlet cross-section and temperature at the other side remains on a lower level, while under the DNB conditions, wall temperature around the outlet cross-section jumps almost simultaneously. When the pressure is very close to the critical point, there is a constant pressure region where the wall temperature rises gently and the CHF no longer exists. The effects of the mass flux and inlet subcooling on CHF have also been discussed. Based on the experiment, a correlation applied for the pressure region close to the critical pressure was proposed to estimate the CHF in the horizontal helically-coiled tube. [ABSTRACT FROM AUTHOR]

Pressure drop is experimentally investigated inside parallel microchannels during subcooled flow boiling of R134a in horizontal orientation. The test conditions included the inlet pressure, the inlet subcooled degree, the heat flux, the vapor quality, and the mass velocity, ranging from 600 to 900 kPa, 1 to 20 K, 5 to 220 kW m −2 , 0 to 95% and 250 to 1000 kg m −2  s −1 , respectively. The effect of the mass velocity and the inlet pressure were investigated. The relative weight of the pressure drop due to two-phase flow acceleration and the friction pressure drop for single phase and two-phase flows were considered. The experimental results for the pressure drop were compared with those predicted by the homogenous model and five other semi-empirical models. [ABSTRACT FROM AUTHOR]

The focus of this paper is heat transfer and pressure drop during condensation in a horizontal smooth round tube. R134a in 6.1 mm inner diameter is used as an example for heat transfer measurements operated in a range of mass fluxes from 50 kg m −2 s −1 to 200 kg m −2 s −1 and heat fluxes from 5 kW m −2 to 15 kW m −2 . The paper also presents pressure drop measurements with mass flux from 100 kg m −2 s −1 to 200 kg m −2 s −1 , showing the effect of mass flux and heat flux on the heat transfer coefficient (HTC) and pressure drop. All the measurements are taken at a constant pressure of 1.319 MPa, which corresponds to a saturation temperature of 50 °C. By connecting heat transfer results with flow characterizations, the behavior of HTC is explained from the perspectives of both mathematics and physics. The result shows that for fixed heat flux and different mass flux, even though the liquid film for higher mass flux is much thinner in the condensing superheated (CSH) region, the HTC is not affected by mass flux. In the two-phase (TP) region, however, higher mass flux clearly yields higher HTC. Opposite behavior is found when heat flux is varied and mass flux is fixed. In the CSH region, HTC increases with heat flux, while in TP region, HTC does not change with heat flux. For both cases, a peak of HTC is presented at quality one, which seems to imply some counter-acting factors that always put heat transfer largest at quality one. The counter-acting factors, however, should not exist based on flow characterization. After mathematically explaining the behavior of HTC in CSH region, film HTC (HTC f ) is proposed and ready to serve as a tool in a unified heat transfer model throughout the entire CSH and TP regions. The result of film HTC shows that the film HTC goes to infinite at the onset of condensation, which is physically correct. Also, the film HTC increases with increasing mass flux. The effect of heat flux, or wall temperature on film HTC is still under discussion. In this study, higher heat flux yields lower film HTC only at high qualities, which is due to the difference in void fraction. In addition, the result of pressure drop measurement also shows that the presence of liquid film tends to increase the pressure drop, which suggests that the wavy structure of vapor–liquid interface increases the friction of the flow. The peak of the pressure drop, however, occurs at lower enthalpy for higher mass flux. The reason could be the later onset of condensation and thinner film, which delays the peak of vapor–liquid interaction at higher mass flux. [ABSTRACT FROM AUTHOR]

In this study, condensation of pure refrigerant R134a vapor inside a vertical 18° helical microfin tube was experimentally investigated. Tests were performed at saturation pressure of 5.7–5.9 bar with mass fluxes of 20–100 kg/m2s and heat fluxes of 1.7–5.3 kW/m2. The effects of mass flux and the temperature difference between the refrigerant and tube wall (ΔT) on the heat transfer performance were analyzed throughout experimental data. For experiments in which ΔT is more than 2.5°C, the average condensation Nusselt number showed a tendency to be independent from ΔT. Heat transfer enhancement ratio was found to be 1.59–1.71, which is always higher than the heat transfer area enhancement factor (1.55). Fins always act as a turbulence promoter in the given experimental data range. Finally, the most widely used heat transfer coefficient correlations for condensation inside microfin tubes were analyzed through the experimental data. Best fit was obtained with Yu and Koyama's correlation with an absolute mean deviation of 17% and Kedzierski and Goncalves's correlation with an absolute mean deviation of 19%. [ABSTRACT FROM PUBLISHER]

The condensation heat transfer characteristics of R134a flowing in a circular tube and 3 flattened copper tubes are investigated. The flattened tubes were made from round tubes with a 3.51 mm inner diameter. The tested tube configurations were as follows: a circular tube with a 3.51 mm inner diameter; flattened tube with a 0.72 aspect ratio (FT1); flattened tube with a 3.49 aspect ratio (FT2); and flattened tube with a 7.02 aspect ratio (FT3). The experimental ranges covered a mass flux of 350–900 kg/m 2 s, heat flux of 10–50 kW/m 2 , inlet quality of 0.1–0.9, and saturation pressure of 8–12 bars. The flow pattern map was initially investigated by comparing it with existing well-known flow pattern maps. Except for some of the results obtained from FT1, most of the experimental data for all of the tested tubes fell into the categories of semiannular and annular flow patterns. The results revealed that the condensation heat transfer coefficient increased with increasing mass flux, heat flux, and vapor quality. The heat transfer coefficients of the flattened tubes were higher than that of the circular tube, around 5–10%, 10–50%, and 200–400% for FT1, FT2, and FT3, respectively. The existing correlations for predicting the heat transfer coefficient were not successful for the flattened tubes. A new correlation was proposed for practical applications. [ABSTRACT FROM AUTHOR]

The pressure-drop characteristics during flow boiling in a single rectangular micro-channel with hydraulic diameter of 0.68 mm are presented. In the present study, pressure drop was studied at heat flux range of 7.63–49.46 kW/m 2 , mass flux range of 600–1400 kg/m 2 s, and saturation temperature of 23, 27 and 31 °C. Experimental results indicated that the total pressure was dominated by frictional pressure drop. The increase of mass flux also increased the frictional pressure gradient, whereas the increase of saturation temperature reduced the frictional pressure gradient. In addition, heat flux also had an insignificant effect on the frictional the pressure gradient. A new correlation was also proposed for effective design of micro-channel heat exchanger. [ABSTRACT FROM AUTHOR]

A series of experiments on flow boiling heat transfer coefficient of R134a in three horizontal circular smooth copper tubes with inner diameters of 1.002, 2.168, and 4.065 mm were conducted. A total of 397 experimental data points are obtained for a mass flux range of 185–935 kg/m 2 s, a heat flux range of 18.0–35.5 kW/m 2 , a saturation pressure range of 0.578–0.82 MPa, and a vapor quality range of 0.03–1.0. The effects of heat flux, saturation pressure, vapor quality, mass flux, and diameter on the heat transfer coefficient are analyzed and the results indicate that the heat transfer coefficient is higher for smaller tubes and for higher saturation pressure while the effects of heat flux, mass flux, and vapor quality are different for different tubes. There are 28 existing correlations of flow boiling heat transfer compared with the experimental data, and the comparison results indicate that the best correlation has a mean absolute deviation of 12.1% and has strong ability to trace the tendency of the heat transfer coefficient with vapor quality. Besides, an experimental database of R134a flow boiling heat transfer is complied, which contains 3000 data points, including 397 from the current experiments and 2603 from 21 published papers. The 28 existing correlations are evaluated with the database, providing a guide for selecting a suitable correlation for specific applications of R134a flow boiling heat transfer. [ABSTRACT FROM AUTHOR]

Condensation heat transfer of R134a which flowed in the inner tube and cooling water which flowed in the annulus were experimentally investigated inside a smooth tube-in-tube heat exchanger in author’s previous studies. In this current work, the investigational setup consists of a 0.5 m long double tube and the inner tube is made up of smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The experiments are carried out for the mass flux range of 260-515 kg m - 2 s - 1 and the heat flux range of 11.3-55.3 kW m - 2 and also at a mean saturation temperature interval of 40-50 °C. The mean heat transfer coefficient of R134a is found through executing an energy equivalence predicated on the energy flow from the test section. The predictability of actual correlations in open sources proposed for two-phase annular flow are tested by using authors’ experimental database. Alterations of vapor quality and saturation temperature difference with calculated condensation heat transfer coefficients are also shown according to different condensing temperatures and mass fluxes. Moreover, the independency of annular flow heat transfer empirical equations from tube orientation and the overall practicality for a vertical tube is validated. [ABSTRACT FROM AUTHOR]

Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf in a vertical stainless steel test section (1.60 mm inside diameter and 245 mm heated length) under upward flow conditions are reported in this article. The experiments were carried out at 27 and 32 °C saturation temperatures with five mass fluxes in the range of 100–500 kg/m 2 s while the applied heat flux was in the range of 5–130 kW/m 2 . The experiments were carried out with gradual increase of the applied heat flux til completion of dryout. Under similar conditions, tests were repeated with R134a in the same test setup to compare thermal performance of these two refrigerants. The results showed that boiling heat transfer was strongly controlled by the applied heat flux and operating pressure with insignificant dependence on mass flux and vapor quality. The frictional pressure drop increased with mass flux and vapor quality and decreased with increasing saturation temperature as expected. Signs of dryout first appeared at vapor qualities of 85%, with the values generally increasing with increasing mass flux. The effect of varying system pressure was insignificant. The experimental results (boiling heat transfer, pressure drop and dryout heat flux) were compared with the predictions from well-known correlations (for macro and micro-scale channels) from the literature. [ABSTRACT FROM AUTHOR]

Pool boiling and condensing heat transfer of R134a on one plain and three enhanced surfaces are experimentally investigated. The saturation temperature in pool boiling is 6 °C and condensing is 40 °C. The heat flux ranges from 8 to 86 kW/m 2 . The enhanced tubes include integral-fin, pyramid and re-entrant cavity surface. The outside diameter of test tubes is 19 mm and the length of test section for boiling is 1100 mm and condensing is 1800 mm. Integral-fin tube has lower heat transfer coefficient in boiling and condensing. The deviations of experiment result and Owen or Webb models are within ±10% for integral-fin tube in condensing. Pyramid surface provides quite close heat transfer coefficient with re-entrant cavity surface in pool boiling and condensing at heat flux greater than 70 kW/m 2 . The heat transfer performances of re-entrant cavity surface tube is the highest among the three enhanced tubes in either pool boiling or condensing at not high heat flux. The heat transfer coefficients can be 1.9–4.8 and 14.8–19.3 times those of a plain tube in pool boiling and condensing respectively. The decreasing rate of heat transfer coefficient for re-entrant cavity surface is also higher than pyramid surface in condensing. Literature survey on nucleate pool boiling and filmwise condensation is also conducted. [ABSTRACT FROM AUTHOR]

The flow patterns during flow boiling of R134a in a horizontal microfin tube have been measured optically in a contiguous glass tube. The microfin tube is made of copper with a total fin number of 60 and a helix angle of 18 ° . The fin height is 0.25 mm and the inner tube diameter at fin root is 8.72 mm. The copper tube is 0.5 m long. It is heated electrically. The glass tube is made of borosilicate glass. The inner tube diameter is 8.8 mm. The experiments have been conducted at the saturation temperature 10 ° C and the heat flux 10 , 000   W m 2 . The mass flux has been varied between 15 and 300   kg s m 2 in steps of 5 and 10 kg s m 2 , respectively. The vapor quality in the glass tube has been varied between 0.1 and 0.9 in steps of 0.1 . The optical measurements based on the different intensity of reflected laser light at the phase interfaces. No reflections have been detected at the glass–liquid interface while there have been reflections at the glass–vapor interface. The laser source has been rotated by 360 ° around the glass tube. Every 2.25 ° measurements of the intensity have been made. Because of the different wetting behavior flow pattern maps for smooth tubes are not suitable for determining the flow patterns in microfin tubes. Due to the helical structure of the fins there is a new flow pattern, the helix flow, where the liquid phase flows helically through the microfin tube. Because of the large wetted surface this leads to an increase of the heat transfer coefficient. The transition curves for a new flow pattern map for microfin tubes are being derived. [ABSTRACT FROM AUTHOR]

An experimental investigation on two phase flow evaporative heat transfer of refrigerants R134a and R407C in a smooth copper tube inclined at five different angles between 0° and 90° was conducted. The experimental data were obtained over a mass velocity range of 100–300kg/m2 s, heat flux range of 3–10kW/m2, inlet temperature range of 5–9°C and vapor quality varied from 0.1 to 0.9. The test section was 1.2m long, smooth copper tube with inner diameter of 7.0mm and outside diameter of 9.52mm. The effects of mass velocity, heat flux, and vapor quality and tube inclinations on evaporative heat transfer coefficient of both refrigerants are thoroughly compared. The experimental heat transfer coefficients were also compared with some existing correlations. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer of R1234yf, R1234ze(E) and R134 was investigated under high heat flux conditions in multi-microchannels. • An infrared camera was used to obtain the flow-wise heat transfer coefficients. • High-speed flow visualization was coupled with the heat transfer measurements. • Three boiling regimes were observed depending on the effect of the wall heat flux and mass flux on the local heat transfer coefficient. • The experimental data were verified against predictive methods. Flow boiling of refrigerants in narrow channels has a great potential to cope with future challenges in the thermal management of high heat flux electronics. This study investigated the flow boiling heat transfer of R1234yf, R1234ze(E), and R134a in multi-microchannels as the applied heat flux was varied from boiling incipience to the critical heat flux. A parametric analysis on the two-phase local heat transfer coefficients was conducted for channel mass fluxes from 415 kg / m 2 s to 1153 kg / m 2 s and saturation temperatures of 30.5 ∘ C and 40.5 ∘ C. The test section comprised 17 channels with inlet restrictions. The channels were 298 µm wide, 1176 µm deep and 10 mm long. Three boiling regimes (I, II, III) were identified according to the observed effect of the heat and mass flux. In boiling regime I, the two-phase local heat transfer coefficient was independent of the mass flux and increased with the heat flux. In boiling regime II, the flow departed from this behavior, and a significant effect of the mass flux appeared. Finally, in boiling regime III, the two-phase local heat transfer coefficient decreased steeply with the heat flux. Flow visualization showed that a predominantly bubbly flow was present in the channels during boiling regime I. In boiling regime II, churn flow was the predominant flow regime, but appeared with more frequent vapor slugs as the heat flux was increased, eventually leading to alternating churn and wispy-annular flow. In boiling regime III, the flow regime alternated between churn and wispy-annular, with a dryout incipience leading to the progressive decrease of the two-phase local heat transfer coefficient until the achievement of the critical heat flux. The experimental data for boiling regimes I and II, amounting to 2504 local values, were compared against the prediction of selected literature correlations. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents experimental results for local heat transfer coefficients, adiabatic frictional pressure gradients and two-phase flow regimes with the low-GWP refrigerant R-1234ze(E), compared to Hydro-Fluoro-Carbon refrigerant R-134a in the same conditions. In particular the results refer to an experimental investigation carried out in a circular smooth tube of 6.00 mm of inner diameter, for saturation temperatures between −2.9 °C and 12.1 °C, mass fluxes between 146 and 520 kg m−2 s−1 and heat fluxes between 5.0 and 20.4 kW m−2. These experimental results are compared to those for R-134a at the same operating conditions. Moreover, an assessment of predictive methods is provided for local heat transfer coefficients and frictional pressure gradients; also a direct comparison of flow regimes visualizations for R-1234ze(E) with a flow pattern map available in literature is presented. [Copyright &y& Elsevier]

Abstract: This paper experimentally investigates the effects of tube and coil diameters on flow boiling heat transfer coefficients inside small diameter (<3 mm) helically coiled tubes. Four different helical coils with coil diameters ranging from 30 mm to 60 mm were studied using tube diameters ranging from 1.1 to 2.8 mm. The heat flux varied from 2500 to 12,000 Wm−2 and mass velocities from 100 to 450 kgm−2s−1. Results have shown that decreasing the tube diameter improved the heat transfer coefficient by up to 63% but early dryout (vapour quality as low as 0.4) was observed especially at high heat fluxes (q = 12,000 Wm−2). Also, decreasing the coil diameter improves the heat transfer coefficient by up to 150%. New correlation for the boiling heat transfer coefficient inside small tube diameter helical coils was developed which predicted the 227 data points used with mean absolute relative error (MARE) of 16%. [Copyright &y& Elsevier]

Abstract: Condensation heat transfer of low GWP refrigerant HFO1234yf was measured in a horizontal tube (inner diameter: 4mm) at a mass flux range of 100–400kgm−2 s−1 and different saturation temperatures (40, 45, and 50°C), and the results were compared with that of R134a and R32. Effects of mass flux, vapor quality, saturation temperature, and thermophysical properties on the heat transfer coefficient were analyzed. Mass flux and vapor quality were presented to primarily affect the heat transfer coefficient in shear-force dominated flow regimes, whereas the thermal conductivity and density ratio are the primary parameters as thermophysical properties influencing the heat transfer coefficient. Observed annular flow regimes agreed with Tandon’s flow pattern map. The measured pressure drop compared with that predicted by the Lockhart–Matinelli correlation, Huang correlation and Haraguchi correlation. And, when comparing the experimental heat transfer coefficient with four heat transfer coefficient correlations, the Haraguchi correlation fairly agreed with the experimental data, with a 10.8% mean deviation. [Copyright &y& Elsevier]

The two-phase heat transfer coefficients of R404A and R134a in a smooth tube of 7.49-mm inner diameter were experimentally investigated at low heat and mass flux conditions. The test section is a 10-m-long counter-flow horizontal double-tube heat exchanger with refrigerant flow inside the tube and hot fluid in the annulus. The heat transfer coefficients along the length of the test section were measured experimentally under varied heat flux conditions between 4 and 18 kW m−2 and mass flux ranging between 57 and 102 kg m−2 s−1 (2.5 to 4.5 g s−1) for saturation temperatures of −10°C, −5°C, and 0°C. The saturation temperatures correspond to pressures of 4.4, 5.2, and 6.1 bar for R404A and 2.0, 2.4, and 3.0 bar for R134a, respectively. The results showed that under the tested conditions, the contribution of the nucleate boiling mechanism is predominant in the heat transfer coefficient throughout the flow boiling process. The Kattan–Thome–Favrat flow pattern maps confirm the occurrence of stratified and stratified-wavy flow patterns for all of the tested conditions. The average heat transfer coefficient of R404A is estimated to be 26 to 30% higher than that of R134a for the same saturation temperature. [ABSTRACT FROM AUTHOR]

Abstract: New correlations of the two-phase multiplier and heat transfer coefficient of R134a during evaporation in a multiport minichannel at low mass flux are proposed. The experimental results were obtained from a test using a counter-flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Test section is composed of the extruded multiport aluminium inner tube with an internal hydraulic diameter of 1.2mm and an acrylic outer tube with an internal hydraulic diameter of 25.4mm. The experiments were performed at heat fluxes between 10 and 35kW/m2, and a refrigerant mass flux between 45 and 155kg/(m2 s). Some physical parameters that influenced the frictional pressure drop and heat transfer coefficient are examined and discussed in detail. The pressure drop and heat transfer coefficient results are also compared with existing correlations. Finally, new correlations for predicting the frictional pressure drop and heat transfer coefficient at low mass fluxes are proposed. [Copyright &y& Elsevier]

Abstract: In the present study, the pressure drop characteristics during the boiling of R134a as it flows through the multiport minichannel heat exchangers are presented. The heat exchangers are counter flow tube-in-tubes. The refrigerant flow runs through the inner tube, while the hot water flows through the 25.4mm round tube at outer edge of the heat exchanger. Two inner test sections are made from aluminium tubing with hydraulic diameters of 1.1mm for 14 channels and 1.2mm for eight channels. The experiments were carried out at mass fluxes ranging from 350 to 980kg/(m2 s), heat fluxes ranging from 18 to 80kW/m2, and saturation pressures of 4, 5 and 6bar, at constant inlet quality of 0.05. The effect of relevant parameters on the frictional pressure drop, such as the saturation pressure, mass flux, heat flux, and aspect ratio, are examined and discussed in detail. Finally, the nine existing correlations that are based on both conventional tubes and small tubes are selected for comparison with the experimental results. [Copyright &y& Elsevier]

Abstract: In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22mm and 1.70mm and a fixed heated length of 220mm. R134a was used as working fluid. Mass flux was varied from 50kg/m2 s to 600kg/m2 s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27°C and 32°C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested. [Copyright &y& Elsevier]

Abstract: Experimental condensation heat transfer data for the new refrigerant R1234ze(E), trans-1,3,3,3-tetrafluoropropene, are presented and compared with refrigerants R134a and R236fa for a vertically aligned, aluminum multi-port tube. Local condensation heat transfer measurements with such a multi-microchannel test section are very challenging due to the large uncertainties related to the heat flux estimation. Presently, a new experimental test facility was designed with a test section to directly measure the wall temperature along a vertically aligned aluminum multi-port tube with rectangular channels of 1.45mm hydraulic diameter. Then, a new data reduction process was developed to compute the local condensation heat transfer coefficients accounting for the non-uniform distribution of the local heat flux along the channels. The condensation heat transfer coefficients showed the expected decrease as the vapor quality decreased (1.0–0.0) during the condensation process, as the mass velocity decreased (260–50kgm−2 s−1) and as the saturation temperature increased (25–70°C). However, the heat transfer coefficients were not affected by the condensing heat flux (1–62kWm−2) or by the entrance conditions within the tested range. It was found that the heat transfer performance of R1234ze(E) was about 15–25% lower than for R134a but relatively similar to R236fa. The experimental data were then compared with leading prediction methods from the literature for horizontal channels. In general, the agreement was poor, over-predicting the high Nusselt number data and under-predicting the low Nusselt number data, but capturing the mid-range quite well. A modified correlation was developed and yielded a good agreement with the current database for all three fluids over a wide range of operating conditions. [ABSTRACT FROM AUTHOR]

Abstract: This paper quantifies the influence of Al2O3 nanoparticles on the pool-boiling performance of R134a/polyolester mixtures on a roughened, horizontal, flat surface. The nanoparticles enhanced the boiling heat transfer relative to that for R134a/polyolester mixtures without nanoparticles for the three lubricant mass fractions that were tested. For the 0.5% nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) as large as 400% for the lowest heat flux. The average heat flux improvement for heat fluxes less than 40 kW m−2 was approximately 105%, 49%, and 155% for the 0.5%, the 1%, and the 2% mass fractions, respectively. A semi-empirical model was developed to predict the boiling enhancement as caused by the interaction of the nanoparticles with the bubbles. The model suggests that small particle size and large nanoparticle volume fraction improve boiling enhancement. [Copyright &y& Elsevier]

Abstract: Critical heat flux (CHF) experiments were performed to study the R134a CHF characteristics in horizontal helically-coiled tubes. The stainless steel test sections were heated uniformly, with tube inner diameters of 3.8–11mm, coil diameters of 135–370mm, helical pitches of 40–105mm and heated lengths of 0.85–7.54m. The experimental conditions are pressures of 0.30–1.10MPa, mass fluxes of 60–480kg m−2 s−1, inlet qualities of −0.32–0.36 and heat fluxes of 6.0×103–9.0×104 Wm−2. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF values decrease with increasing heated lengths, coil diameters and inner diameters, but the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/d i >200. The coil-to-diameter ratios are more important than length-to-diameter ratios for CHF in helically-coiled tubes, while the helical pitches have little effect on CHF. The CHF value increases greatly with increasing mass flux and decreases smoothly with increasing pressure. It decreases nearly linearly with increasing inlet and critical qualities, but it varies more acutely at x cr <0.5 than higher critical qualities. New correlations for R134a flow boiling CHF in horizontal helically-coiled tubes were developed for applications. [ABSTRACT FROM AUTHOR]

Abstract: The flow boiling heat transfer characteristics of R134a in the multiport minichannel heat exchangers are presented. The heat exchanger was designed as the counter flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Two inner tubes were made from extruded multiport aluminium with the internal hydraulic diameter of 1.1mm for 14 numbers of channels and 1.2mm for eight numbers of channels. The outer surface areas of two inner test sections are 5979mm2 and 6171m2, while the inner surface areas are 13,545mm2 and 8856mm2 for 14 and eight numbers of channels, respectively. The outer tube of heat exchanger was made from circular acrylic tube with an internal hydraulic diameter of 25.4mm. The experiments were performed at the heat fluxes between 15 and 65kW/m2, mass flux of refrigerant between 300 and 800kg/m2 s and saturation pressure ranging from 4 to 6bar. For instance the boiling curve, average heat transfer coefficients are discussed. The comparison results of two test sections with different the number of channels are investigated. The results are also compared with nine existing correlations. The new correlation for predicting the heat transfer coefficient was also proposed. [Copyright &y& Elsevier]

Abstract: An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10mm, inner diameter of 8.4mm, coil diameter of 300mm, helical pitch of 75mm and valid heated length of 1.89m. The experimental parameters are the outlet pressures of 0.30–0.95MPa, mass fluxes of 60–500kgm−2 s−1, inlet qualities of −0.36–0.35 and heat fluxes of 7.0×103–5.0×104 Wm−2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270° and 90°) of the outlet cross-section. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of R134a heat transfer coefficients and wall temperature distribution were investigated under low mass flux and low pressure conditions in a helically coiled tube with heated length of 7070mm, outer diameter of 10mm, inner diameter of 7.6mm, coil diameter of 300mm and helical pitch of 40mm. System pressures, mass fluxes and inlet qualities range from 0.20 to 0.75MPa, 50 to 260kg/m2 s and −0.18 to 0.40, respectively. It was found that the wall temperatures in descending segments of coiled tube were higher than those of climbing ones, while the heat transfer coefficients varied inversely. Around the section circumference, the outside temperature was lower than the inside one; this is more apparent at very low mass flux and pressure conditions. The heat transfer coefficient increases with increasing mass flux, vapor quality and heat flux. However, the pressure has an indeterminate effect. New heat transfer coefficient correlations for current conditions were developed comparing with existing correlations. [ABSTRACT FROM AUTHOR]

Abstract: Heat transfer during reflux condensation in a narrow tube (0.007m inner diameter, 0.5m length) and a rectangular channel (0.007m hydraulic diameter, 0.5m length) was investigated. The basic mechanism of this process, i.e. removing high boiling point components from a stream of a more volatile fluid or even from inert gases, is still not sufficiently understood. The study presents experimental results with the zeotropic mixture of R134a and R123 at a constant composition of the incoming vapour of 50mol%. The results show that both, gas side heat and mass transfer resistances and the resistances in the liquid film influence the condensation process. Channel inclination is less important in mixture condensation than in pure fluid condensation but not negligible, and increasing vapour mass flow enhances mixture condensation. Contrary to pure fluid results heat transfer in the rectangular channel with the same hydraulic diameter is much smaller than in the tube at the same vapour mass flow rate. [Copyright &y& Elsevier]

Abstract: In the present study, new experimental data are presented for literature on the prediction of film thickness and identification of flow regime during the co-current downward condensation in a vertical smooth copper tube having an inner diameter of 8.1mm and a length of 500mm. R134a and water are used as working fluids in the tube side and annular side of a double tube heat exchanger, respectively. Condensation experiments are done at mass fluxes of 300 and 515kgm−2 s−1. The condensing temperatures are between 40 and 50°C; heat fluxes are between 12.65 and 66.61kWm−2. The average experimental heat transfer coefficient of the refrigerant HFC-134a is calculated by applying an energy balance based on the energy transferred from the test section. A mathematical model by Barnea et al. based on the momentum balance of liquid and vapor phases is used to determine the condensation film thickness of R134a. The comparative film thickness values are determined indirectly using relevant measured data together with various void fraction models and correlations reported in the open literature. The effects of heat flux, mass flux, and condensation temperature on the film thickness and condensation heat transfer coefficient are also discussed for the laminar and turbulent flow conditions. There is a good agreement between the film thickness results obtained from the theoretical model and those obtained from six of 35 void fraction models in the high mass flux region of R134a. In spite of their different valid conditions, six well-known flow regime maps from the literature are found to be predictive for the annular flow conditions in the test tube in spite of their different operating conditions. [Copyright &y& Elsevier]