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Highlights • Heat transfer and pressure drop of R-134a in dimpled tubes are investigated. • The effects of dimpled depth on the heat transfer and pressure drop are examined. • The heat transfer enhancement factor is up to 1.81 times of smooth tube. • The correlations to predict the Nusselt number and friction factor are proposed. Abstract An experimental investigation is conducted to determine the effect of dimpled depth on the condensation heat transfer coefficient and pressure drop of R-134a flowing inside dimpled tubes. The test condenser is a double tube heat exchanger where the refrigerant flows inside and water flows in the annulus. The inner tube is a 1500 mm long and 8.1 mm inside diameter. The experiments are carried out for one smooth tube and three dimpled tubes having dimpled depth of 0.5, 0.75, and 1.0 mm. For each test tube, several test runs are performed over mass flux range of 300–500 kg/m2s, heat flux range of 10–20 kW/m2, and condensing temperature range of 40–50 °C. The experimental results reveal that the dimpled tube presents the significant heat transfer enhancement and pressure drop penalty. The tube with the highest dimpled depth yields the highest heat transfer enhancement and pressure drop penalty up to 83% and 892% higher than those of the smooth tube, respectively. Additionally, the overall performance of dimpled tube is evaluated in term of efficiency index. The new correlations including the effect of dimpled depth, dimpled pitch, and helical pitch on the Nusselt number and friction factor of R-134a in dimpled tube are developed. [ABSTRACT FROM AUTHOR]

The heat transfer enhancement and pressure loss penalty of condensing refrigerant R-134a in dimpled tubes with different helical and dimpled pitches are experimentally investigated. The test section is a 1500-mm-long tube-in-tube heat exchanger placed horizontally. The inside and outside diameters of the inner tubes are 8.1 and 9.52 mm, respectively. One smooth tube and five dimpled tubes with different helical and dimpled pitches are tested. The experimental runs are done at saturated condensing temperatures ranging from 40 to 50 °C. The mass fluxes and heat fluxes vary from 300 to 500 kg/m 2  s and from 10 to 20 kW/m 2 , respectively. The results show that the dimpled tubes yield higher heat transfer enhancement and pressure drop penalty compared to the smooth tube under the same operating condition. By decreasing helical and dimpled pitches, both the heat transfer coefficient and frictional pressure drop increase significantly. The maximum heat transfer coefficient and frictional pressure drop are achieved through the tube with the lowest helical and dimpled pitches (p = 5.08 mm, z = 3.24 mm) up to 88% and 630% higher than those of the smooth tube, respectively. The new correlations for predicting the Nusselt number and friction factor of R-134a condensing in dimpled tubes with different helical and dimpled pitches are proposed. [ABSTRACT FROM AUTHOR]

This paper presents the condensation heat transfer of R-134a flowing inside a chevron gasketed plate heat exchanger. The study focuses on the effect of surface roughness on the heat transfer coefficient and pressure drop. The stainless steel plate heat exchanger has a total length of 360 mm and total width of 100 mm. The experiment is conducted at a condensation temperature of 40–50 °C, a heat flux of 5–15 kW/m 2 , and refrigerant mass flux of 61–89 kg/m 2 s and plate surface roughness ranging between 0.594 μm and 2.754 μm. Unlike the increase in temperature, the increase in surface roughness and mass flux leads to an increase in the heat transfer coefficient and frictional pressure gradient. The increase in heat flux leads to a slight increase in heat transfer coefficient, but has no effect on the pressure drop. The thermal performance factors are in the range of 0.99–1.22 and 0.92–1.19 for roughnesses of 1.816 and 2.754 µm, respectively. [ABSTRACT FROM AUTHOR]

This research reports the thermal performance and flow characteristics of nanofluid flows in two different types of microchannel heat sink (MCHS) with multiple zigzag flow channel structures experimentally with regard to the continuous zigzag flow channel (CZ-HS) and the single cross-cutting zigzag flow channel (CCZ-HS). SiO 2 nanoparticles with particle loadings of 0.3, 0.6, and 0.8 vol.% and dispersed in deionized water (DI water) are used as the working medium. Both CZ-HS and CCZ-HS are made from copper material. Their dimensions are approximately 28 × 33 mm. Hydraulic diameter and number of flow channels are equally designed as seven 1-mm flow channels, respectively. The heat transfer area of CZ-HS is approximately 1176 mm 2 and that of CCZ-HS is 1238 mm 2 . The effects of single cross-cutting of the flow channel, Reynolds number, and particle concentration on the Nusselt number and pressure drop characteristics are investigated. The experimental data indicate that the nanofluid-cooled heat sink provided larger thermal performance than the heat sink cooled by water of approximately 3–15%. Similarly, the results indicated that the thermal performances of the CCZ-HS are larger than those of the CZ-HS by an average of 2–6%. For the pressure drop, the measured data showed that particle concentration and cross-cutting of the flow channel have a small effect on the pressure-drop data. [ABSTRACT FROM AUTHOR]

This study investigates the heat transfer and pressure drop of R-134a during condensation inside a dimpled tube. The test section is a horizontal counter-flow double-tube heat exchanger with refrigerant flowing in the inner tube and cold water flowing in the annulus. The inner tubes consist of one smooth tube and one dimpled tube, which are made from copper. The length and inner diameter of the test tube are 1500 mm and 8.1 mm, respectively. The test runs are performed at saturation temperatures of 40, 45, and 50 °C, heat fluxes of 10, 15, and 20 kW/m 2 , and mass fluxes of 300, 400, and 500 kg/m 2 s. The effects of heat flux, mass flux, and saturation temperature on the heat transfer coefficient and frictional pressure drop are examined. Comparisons between smooth and dimpled tubes on the heat transfer and frictional pressure drop are also discussed. It is observed that the heat transfer coefficient and frictional pressure drop obtained from a dimpled tube are higher than that of the smooth tube. In addition, as the equivalent Reynolds number increases, the dimpled tube enhances the Nusselt number around 1.3–1.4 times in comparison to the smooth tube. Whereas, when the equivalent Reynolds number decreases, the two-phase friction factor rises around 2.8–4.1 times as compared to the smooth tube. [ABSTRACT FROM AUTHOR]

The effects of operating conditions on the air-side heat transfer, and pressure drop of a micro-channel heat exchanger under wet surface conditions were studied experimentally. The test section was an aluminum micro-channel heat exchanger, consisting of a multi-louvered fin and multi-port mini-channels. Experiments were conducted to study the effects of inlet relative humidity, air frontal velocity, air inlet temperature, and refrigerant temperature on air-side performance. The experimental data were analyzed using the mean enthalpy difference method. The test run was performed at relative air humidity's ranging between 45% and 80%; air inlet temperature ranges of 27, 30, and 33 0C, refrigerant-saturated temperatures ranging from 18 to 22 0C, and Reynolds numbers between 128 and 166. The results show that the inlet relative humidity, air inlet temperature, and the refrigerant temperature had significant effects on heat transfer performance and air-side pressure drop. The heat transfer coefficient and pressure drop for the micro-channel heat exchanger under wet surface conditions are proposed in terms of the Colburn factor and Fanning factor. [ABSTRACT FROM AUTHOR]

A two-phase air–water flow experiment was carried out to obtain flow pattern, void fraction, and pressure drop data from a vertical micro-channel, which was a fused silica tube with a diameter of 0.53 mm and a length of 320 mm. The adiabatic two-phase flow behaviors during vertical upward direction were experimentally investigated, and consequently, a buoyancy effect can be detected in this work. A flow visualization study was performed, leading to observation of slug flow, throat–annular flow, churn flow, annular flow, and annular–rivulet flow. The shape of the interfacial surface in the slug flow pattern was deformed during vertical upward flow, which was different from the bullet-shaped gas slug observed in the horizontal channel. The flow visualization results also indicate that the flow pattern map for vertical upward flow is not completely compatible with that for horizontal flow. Image analysis was performed to determine the void fraction, which increases linearly with increasing volumetric quality. The vertical upward flow gave a lower void fraction than the horizontal flow. The frictional pressure drop can be increased when the churn flow is formed in the channel. In addition, vertical upward flow can result in higher pressure drop when compared with the horizontal channel. [ABSTRACT FROM AUTHOR]

The paper reports an experimental investigation into the thermal performance and pressure drop characteristics of nanofluid-cooled heat sinks and then compares the results with the data for water-cooled heat sinks. Heat sinks with miniature circular-fin (MCFHS) and square-fin (MSFHS) structures are used and made from aluminum material with dimensions of 28 × 33 mm. Similarly, SiO 2 nanoparticles dispersed in deionized water with particle concentrations of 0.2%, 0.4%, and 0.6% volume are used as working fluids. The effects of pin fin configuration, particle concentration, and flow rate on the heat transfer performance and flow behaviors are presented. Reynolds numbers based on the hydraulic diameter of each flow channel ranging between 700 and 3700, fluid temperature of 15 °C, and heat flux ranging from 2 and 5 W/cm 2 are tested. Hydraulic diameters based on each flow channel are equally designed at 1.2 mm for both heat sinks. The experimental results indicate that the heat transfer coefficient increased with increasing Reynolds numbers and particle concentrations. The MCFHS gave greater heat transfer performance than that of the MSFHS by about 6–9%. For pressure drop data, the measured data showed that the pin fin configuration and particle concentration had small effects on the pressure drop and pumping power. [ABSTRACT FROM AUTHOR]

Experimental data regarding heat transfer coefficient and pressure drop of water flow inside a plate heat exchanger with a rough surface is investigated and compared with that obtained from a smooth surface. Three commercial stainless steel corrugated plates with symmetrical chevron angle of 25° are used. The water flow inside the plate heat exchanger is arranged for a single pass and counter flow. The test runs are performed at Reynolds numbers ranging between 1300 and 3200 and plate surface roughness ranging between 0.936 μm and 3.312 μm. The experimental results show that increase in surface roughness yields an increase in heat transfer coefficient between 4.46% and 17.95% and an increase in pressure drop between 3.90% and 19.24% with respect to a smooth surface. The correlations for predicting the Nusselt number and friction factor of water flow inside a plate heat exchanger with a rough surface are proposed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effect of the free rotation of a swirl device is investigated. [•] Five swirl generators are installed equally distant along the test section. [•] Data of fixed turbine and freely rotating turbine are compared. [•] The performance for rotating turbine is 6.3% higher than that for fixed turbine. [•] The tube with RTSG inserts gave the lowest pressure drop. [Copyright &y& Elsevier]

Abstract: Heat transfer enhancement and flow behaviors in horizontal circular tubes installed with five rotating turbine-type swirl generators (RTSG) are presented experimentally. This type of RTSG is an innovative design that has not been seen in the previous research. Deionized water is used as a working fluid and flows through the test section under turbulent flow conditions. A smooth, stainless steel tube 9.2mm in ID, 0.4mm in thickness, and 2.3m in length is used as the test section. RTSG made from aluminum, with a twisted angle of 60° and a length of 2cm, are located at 0.46m equally distant along the test section. When fluid flows through the RTSG, continuously free rotations are observed, and swirl flow after RTSG is created. A DC power supply is used to supply heat load to the test section. Compared with a common, plain tube (without rotating swirl devices), the experimental results indicate that the local heat transfer coefficient of the tube with RTSG inserts is higher than that of the common, plain tube, and they increase when the working fluid flows through the RTSG. Similarly, the wall temperatures of the tube with RTSG inserts are lower than those of the common tube. However, the pressure drop of the tube fitted with RTSG is significantly greater than that of the common, plain tube. [Copyright &y& Elsevier]

Abstract: Adiabatic two-phase air–water flow is experimentally studied in this work. Two channels, made of fused silica, with different diameters of 0.53 and 0.15mm are used as test sections. The void fraction data for both tubes are obtained by image analysis. For the larger channel, the void fraction is found to be a linear relationship with the volumetric quality. In the case of the smaller one, however, the non-linear void fraction is obtained. The measured frictional pressure drop data are compared with the predictions regarding the homogeneous flow assumption. Several well-known two-phase viscosity models are subsequently evaluated for applicability to micro-channels. [Copyright &y& Elsevier]

Abstract: In the present study, the air–water two-phase flow characteristics including flow pattern and pressure drop inside a plate heat exchanger are experimentally investigated. A plate heat exchanger with single pass under the condition of counter flow is operated for the experiment. Three stainless steel commercial plates with a corrugated sinusoidal shape of unsymmetrical chevron angles of 55° and 10° are utilized for the pressure drop measurement. A transparent plate having the same configuration as the stainless steel plates is cast and used as a cover plate in order to observe the flow pattern inside the plate heat exchanger. The air–water mixture flow which is used as a cold stream is tested in vertical downward and upward flow. The results from the present experiment show that the annular-liquid bridge flow pattern appeared in both upward and downward flows. However, the bubbly flow pattern and the slug flow pattern are only found in upward flow and downward flow, respectively. The variation of the water and air velocity has a significant effect on the two-phase pressure drop. Based on the present data, a two-phase multiplier correlation is proposed for practical application. [ABSTRACT FROM AUTHOR]

Abstract: The heat transfer coefficient and pressure drop of R-134a inside a horizontal smooth tube and corrugated tubes are experimentally investigated. The test section is a 2.0m long counter-flow concentric double tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. A smooth tube and corrugated tubes having inner diameters of 8.7mm are used as an inner tube. The corrugation pitches are 5.08, 6.35, and 8.46mm, respectively. The corrugation depth of all corrugated tubes is fixed at 1.5mm. The outer tube is made from smooth copper tube having an inner diameter of 21.2mm. The test runs are performed at the saturation temperatures of 40, 45, and 50°C, heat fluxes of 5 and 10kW/m2, and mass fluxes ranging from 200 to 700kg/m2 s. The results obtained from the corrugated tubes are compared with that of the smooth tube. It is found that the corrugation pitches have a significant effect on the heat transfer coefficient and pressure drop augmentations. [Copyright &y& Elsevier]

Abstract: Two-phase air–water flow characteristics are experimentally investigated in horizontal circular micro-channels. Test sections are made of fused silica. The experiments are conducted based on three different inner diameters of 0.53, 0.22 and 0.15mm with the corresponding lengths of 320, 120 and 104mm, respectively. The test runs are done at superficial velocities of gas and liquid ranging between 0.37–42.36 and 0.005–3.04m/s, respectively. The flow visualisation is facilitated by systems mainly including stereozoom microscope and high-speed camera. The flow regime maps developed from the observed flow patterns are presented. The void fractions are determined based on image analysis. New correlation for two-phase frictional multiplier is also proposed for practical applications. [Copyright &y& Elsevier]

Abstract: This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2vol.% TiO2 nanoparticles. The heat transfer coefficient and friction factor of the TiO2–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO2 nanoparticles of about 21nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop. [Copyright &y& Elsevier]

Abstract: This paper is a continuation of the authors’ previous work. Two-phase air–water flow experiments are performed in a horizontal circular micro-channel. The test section is made of a fused silica tube with an inner diameter of 0.15mm and a length of 104mm. The flow phenomena, which are liquid/unstable annular alternating flow (LUAAF), liquid/annular alternating flow (LAAF), and annular flow, are observed and recorded by a high-speed camera mounted together with a stereozoom microscope. A flow pattern map is presented in terms of the phase superficial velocities and is compared with those of other researchers obtained from different working fluids. Image analysis is performed to determine the void fraction, which increases non-linearly with increasing volumetric quality. It is revealed that the two-phase frictional multiplier data show a dependence on flow pattern rather than mass flux. Based on the present data, a new pressure drop correlation is proposed for practical applications. According to the present study, in general the data for the two-phase air–water flow characteristics are found to comply with those of working fluids other than air–water mixtures. [Copyright &y& Elsevier]

Abstract: The evaporation heat transfer coefficient and pressure drop of R-410A flowing through a horizontal aluminium rectangular multiport mini-channel having 3.48mm hydraulic diameter are experimentally investigated. The test runs are performed at mass flux ranging between 200 and 400kg/m2 s. The heat fluxes are between 5 and 14.25kW/m2 and the saturation temperatures range between 10 and 30°C. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of the imposed wall heat flux, mass flux, vapour quality, and saturation temperature on the evaporation heat transfer and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the existing correlation. New correlations for the evaporation heat transfer coefficient and pressure drop of R-410A flowing through a multiport mini-channel are proposed for practical applications. [Copyright &y& Elsevier]

Abstract: Adiabatic two-phase air–water flow characteristics, including the two-phase flow pattern as well as the void fraction and two-phase frictional pressure drop, in a circular micro-channel are experimentally studied. A fused silica channel, 320mm long, with an inside diameter of 0.53mm is used as the test section. The test runs are done at superficial velocity of gas and liquid ranging between 0.37–16 and 0.005–3.04m/s, respectively. The flow pattern map is developed from the observed flow patterns i.e. slug flow, throat-annular flow, churn flow and annular-rivulet flow. The flow pattern map is compared with those of other researchers obtained from different working fluids. The present single-phase experiments also show that there are no significant differences in the data from the use of air or nitrogen gas, and water or de-ionized water. The void fraction data obtained by image analysis tends to correspond with the homogeneous flow model. The two-phase pressure drops are also used to calculate the frictional multiplier. The multiplier data show a dependence on flow pattern as well as mass flux. A new correlation of two-phase frictional multiplier is also proposed for practical application. [Copyright &y& Elsevier]

Abstract: The two-phase heat transfer coefficient and pressure drop of pure HFC-134a condensing inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786m long helically coiled double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth copper tubing of 9.52mm outer diameter and 8.3mm inner diameter. The outer tube is made from smooth copper tubing of 23.2mm outer diameter and 21.2mm inner diameter. The heat exchanger is fabricated by bending a straight copper double-concentric tube into a helical coil of six turns. The diameter of coil is 305mm. The pitch of coil is 35mm. The test runs are done at average saturation condensing temperatures ranging between 40 and 50°C. The mass fluxes are between 400 and 800kgm−2 s−1 and the heat fluxes are between 5 and 10kWm−2. The pressure drop across the test section is directly measured by a differential pressure transducer. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy rejected from the test section. The effects of heat flux, mass flux and, condensation temperature on the heat transfer coefficients and pressure drop are also discussed. It is found that the percentage increase of the average heat transfer coefficient and the pressure drop of the helically coiled concentric tube-in-tube heat exchanger, compared with that of the straight tube-in-tube heat exchanger, are in the range of 33–53% and 29–46%, respectively. New correlations for the condensation heat transfer coefficient and pressure drop are proposed for practical applications. [Copyright &y& Elsevier]

Abstract: The performance of heat exchangers can be improved to perform a certain heat-transfer duty by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration, etc. The passive techniques require fluid additives or special surface geometries. Curved tubes have been used as one of the passive heat transfer enhancement techniques and are the most widely used tubes in several heat transfer applications. This article provides a literature review on heat transfer and flow characteristics of single-phase and two-phase flow in curved tubes. Three main categories of curved tubes; helically coiled tubes, spirally coiled tubes, and other coiled tubes, are described. A review of published relevant correlations of single-phase heat transfer coefficients and single-phase, two-phase friction factors are presented. [Copyright &y& Elsevier]

Abstract: Flow patterns and pressure drop of upward liquid single-phase flow and air–water two-phase flow in sinusoidal wavy channels are experimentally studied. The test section is formed by a sinusoidal wavy wall of 1.00 m length with a wave length of 67.20mm, an amplitude of 5.76mm. Different phase shifts between the side walls of the wavy channel of 0°, 90° and 180° are investigated. The flow phenomena, which are bubbly flow, slug flow, churn flow, and dispersed bubbly flow are observed and recorded by high-speed camera. When the phase shifts are increased, the onset of the transition from the bubbly flow to the churn flow shifts to a higher value of superficial air velocity, and the regions of the slug flow and the churn flow are smaller. In other words, the regions of the bubbly flow and the dispersed bubbly flow are larger as the phase shift increases. The slug flow pattern is only found in the test sections with phase shifts of 0° and 90°. Recirculating gas bubbles are always found in the troughs of the corrugations. The recirculating is higher when the phase shifts are larger. The relationship between the two-phase multipliers calculated from the measured pressure drops, and the Martinelli parameter is compared with the Lockhart–Martinelli correlation. The correlation in the case of turbulent–turbulent condition is shown to fit the data very well for the phase shift of 0° but shows greater deviation when the phase shifts are higher. [Copyright &y& Elsevier]

Abstract: Two-phase flow pattern, pressure drop and void fraction in horizontal and inclined upward air–water two-phase flow in a mini-gap annular channel are experimentally studied. A concentric annular test section at the length of 880mm with an outer diameter of 12.5mm and inner diameter of 8mm is used in the experiments. The flow phenomena, which are plug flow, slug flow, annular flow, annular/slug flow, bubbly/plug flow, bubbly/slug–plug flow, churn flow, dispersed bubbly flow and slug/bubbly flow, are observed and recorded by high-speed camera. A slug flow pattern is found only in the horizontal channel while slug/bubbly flow patterns are observed only in inclined channels. When the inclination angle is increased, the onset of transition from the plug flow region to the slug flow region (for the horizontal channel) and from the plug flow region to slug/bubbly flow region (for inclined channels) shift to a lower value of superficial air velocity. Small shifts are found for the transition line between the dispersed bubbly flow and the bubbly/plug flow, the bubbly/plug flow and the bubbly/slug–plug flow, and the bubbly/plug flow and the plug flow. The rest of the transition lines shift to a higher value of superficial air velocity. Considering the effect of flow pattern on the pressure drop in the horizontal tube at low liquid velocity, the occurrence of slug flow stops the rise of pressure drop for a short while, before rising again after the air velocity has increased. However, the pressure does not rise abruptly in the tubes with θ =30° and 60° when the slug/bubbly flow occurs. At low gas and liquid velocity, the pressure drop increases, when the inclination angles changes from horizontal to 30° and 60°. Void fraction increases with increasing gas velocity and decreases with increasing liquid velocity. After increasing the inclination angle from horizontal to θ =30° and 60°, the void fraction appears to be similar, with a decreasing trend when the inclination angle increases. [Copyright &y& Elsevier]

Abstract: The two-phase heat transfer coefficient and pressure drop of HFC-134a during evaporation inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786-m long helically coiled tube with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made from copper tubing of 9.52mm outer diameter and 7.2mm inner diameter. The heat exchanger is fabricated by bending a straight copper tube into a spiral coil. The diameter of coil is 305mm. The test run are done at average saturated evaporating temperatures ranging between 10 and 20°C. The mass fluxes are between 400 and 800kgm−2 s−1 and the heat fluxes are between 5 and 10kWm−2. The inlet quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of heat flux, mass flux and, evaporation temperature on the heat transfer coefficients and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the straight tube reported in the literature. New correlations for the convection heat transfer coefficient and pressure drop are proposed for practical applications. [Copyright &y& Elsevier]

Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented in this study. The data was obtained based on flow boiling experiments with R-134a flow through a 1 mm diameter channel which was aligned in different orientations, i.e. horizontal flow, vertical upward flow, and vertical downward flow. A constant surface heat flux condition was performed under a saturation pressure of 8 bar, a heat flux range of 1–60 kW/m 2 , and a mass flux range of 250–820 kg/m 2 s. The experimental results showed the importance of the change in the flow direction. The shape of the gas slug during horizontal flow did not look the same as in the vertical orientations. Heat transfer coefficient and pressure drop became increased when the refrigerant flowed in the vertical downward direction. The experimental data was also compared with the existing prediction methods. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the single phase pressure drop characteristics of smooth and microfin tubes are investigated experimentally. The horizontal test section is a counter flow double tube heat exchanger with water flowing in the inner tube and cooling water flowing in the annulus. By means of experimental setup, required temperature and pressure measurements are recorded and friction factor coefficient and pressure drop of smooth and microfin tubes are determined. Experiments are conducted for mass flow rates in the range between 0.023kg/s and 0.100kg/s and effect of Reynolds number on pressure drop is investigated. By using experimental results, Blasius type friction factor equations are developed for both smooth and microfin tubes. Experimental results for both smooth and microfin tubes are compared with correlations given in the literature. [Copyright &y& Elsevier]

Abstract: New experimental data on the influence of short-tube orifice configuration, including diameter, length, length-to-diameter ratio (L/D), and orientation on the flow pattern, mass flow rate, and pressure distribution of HFC-134a inside the short-tube orifice are presented. Short-tube orifice diameters ranging between 0.605 and 1.2mm with L/D ranging between 1.87 and 33 are used in the experiments. Three different forms of the metastable liquid flow, which are metastable liquid core flow, conical metastable liquid core flow, and full metastable liquid flow are visually observed. The short-tube orifice diameter has a significant effect on the increase in the flow rate. Conversely, the change in the orientation of the test section has no significant effect on the flow rate. The choke flow phenomenon disappears inside the short-tube orifice when L/D is less than 2.91. Based on the present data, a correlation for predicting the mass flow rate through short-tube orifices is proposed. [Copyright &y& Elsevier]

This study aims to evaluate the effect of the concentration of multi-wall carbon nanotubes (MWCNTs)/ turbine meter oil nanofluids on thermophysical attributes of lubricants, including the viscosity index, kinematics viscosity, flash point, pressure drop, and friction factor. The results indicated that the viscosity of pure lubricant was enhanced with the addition of MWCNTs and increased with decreasing temperature. The viscosity index at a concentration of 0.3 wt% was increased by 2.43%. The flash point temperature at concentrations of 0.3 wt% and 0.4 wt% was increased by 4.4%. Experiments were conducted to obtain the pressure drop data and friction factor. The friction factor increased for all concentrations of MWCNTs in pure oil; the highest increment in the friction factor, with the addition of 0.3 wt% nanoparticles to pure oil in 0.069 m/s inlet velocity, was 10.41%. The mean deviation between the experimental friction factor and simulation friction factor was 0.029. Unlabelled Image • The MWCNT/Turbine meter oil nanofluid was synthesized. • The friction coefficient was increased in all of the weight percentages of MWCNTs. • The viscosity index and viscosity were enhanced up to 2.43% and 8.83%. • The flash point of nanofluid increased up to 10 °C in presence of the MWCNT. • The highest difference between the experimental results and the simulation was 10.76% at the velocity inlet of 0.698 m/s. [ABSTRACT FROM AUTHOR]

In order to avoid the costs of experimental evaluations, soft computing methods like artificial neural network (ANN) and genetic algorithm have remarkably grabbed the attentions of investigators for predicting the hydrothermal characteristics of different types of nanofluids. In this paper, the implementation of ANN and genetic algorithm for modeling and multi-criteria optimizing the hydrothermal behavior of SiO 2 /water nanofluid has been investigated. Using the data obtained from the experimental analysis, an ANN model is developed to estimate the pressure drop and Nusselt number as a function of volume concentration, Reynolds number, and inlet temperature. Different network structures were assessed and it has been achieved that a network with 2 hidden layers and 6 neurons in every layer provides the most accurate prediction. It is revealed that the developed network is satisfactorily accurate to determine the Nusselt number and pressure drop of SiO 2 /water nanofluid compared to the empirical correlations. To optimize the hydrothermal behavior of the nanofluid (i.e. to find the optimal cases with highest Nusselt number and the relatively least pressure drop), the genetic algorithm coupled with compromise programming approach has been implemented considering decision maker's attitude. [ABSTRACT FROM AUTHOR]

In the present paper, in order to understand the accuracy of 38 different correlations derived by various researchers in this field, the correlations were executed for condensation frictional pressure drop. To accomplish this goal, experimental data provided from authors' previous publications encompassing 412 points for two smooth tubes, and 334 points for five corrugated tubes, have been utilized so as to compare the determined results. The experimental setup is composed of a 2.5 m double tube for horizontal configuration and smooth and corrugated tubes at the inner diameters of 0.0081 m, while the applied mass flux range spans between 709 and 1974 kg m − 2 s − 1 . The average quality of vapor and saturation pressure ranges lie within 0.09 to 0.97, and 10 to 13 bar, respectively. Determining the frictional pressure drop in two-phase flows does not involve corrugated tube geometry in the calculation of friction factor, to make this available, a slight alteration that requires the replacement of a correlation with another one in the literature was suggested with regard to friction factor approach. As a result of this, it was noticed that performances of some correlations were optimized to predict the frictional pressure drop in corrugated tubes. Additionally, the most effective correlations have been selected for the horizontal double pipe heat exchanger having smooth and corrugated tubes. Finally, alteration of the condensation pressure drop with Reynolds number are presented using both experimental data and best predictive correlations. [ABSTRACT FROM AUTHOR]

In this work, heat transfer and pressure drop characteristics of nanofluids flowing through a horizontal circular tube have been investigated experimentally. The test tube was made of stainless steel type 304 with an inner diameter of 7.1 mm. The working fluid was SiO 2 /water nanofluid where the average diameter of nanoparticles was 7 nm. Nanofluids at three different volume concentrations of 0.5, 1, and 2% have been prepared and tested. The experiments have been performed for Reynolds numbers ranging from 3800 to 12000, inlet temperatures of 25, 30, and 35 °C where a constant heat flux was imposed on the tube. The effects of particle volume concentrations, inlet temperature and mass flow rate on convective heat transfer and pressure drop characteristics have been evaluated. The results revealed that with increasing Reynolds number, volume concentration, and inlet temperature the heat transfer coefficient and Nusselt number increased. Moreover, pressure drop increased with increasing volume concentration; conversely, decreased with increasing inlet temperature. The efficiency index reached its maximum quantity (i.e. 1.6) at Reynolds numbers higher than 9000, the volume concentration of 2%, and inlet temperature of 35 °C. On the other hand, the minimum values of efficiency index were obtained for Reynolds numbers less than 7000, the volume fraction of 0.5%, and inlet temperature of 25 °C. Finally, new correlations for predicting the Nusselt number and friction factor of SiO 2 /water turbulent flow have been proposed. [ABSTRACT FROM AUTHOR]

The study of pulp suspension flow is one of the most significant scientific interests since the considerable changes in the flow behavior of water were noticed after adding a small amount of fiber to water. The rheological studies on Kenaf pulp (a fiber crop) suspensions revealed that concentration has a great influence on variations of viscosity and yielded stress values. Heat transfer and pressure loss data were obtained from Kenaf suspension flow at different concentrations over a wide velocity range in a straight pipe. Kenaf suspension at the concentration of 0.6 wt.% showed maximum drag reduction. A considerable increase in the heat transfer coefficient of Kenaf suspension (0.2 wt.%) was also observed. Such an increment in the heat transfer coefficient ratio of Kenaf suspension flow is interesting since the enhancement of heat transfer has mostly been reported for suspensions containing nanoparticles and not for natural fibers in microscale. Moreover, the effects of fiber length and flexibility on momentum and heat transfer data were studied. [ABSTRACT FROM AUTHOR]

Nitrogen-doped graphene (NDG) nanofluids are prepared using a two-step method in an aqueous solution of 0.025 wt.% Triton X-100 as a surfactant with various nanosheets at several concentrations (0.01, 0.02, 0.04, 0.06 wt.%). This paper reports results of experiments on thermal conductivity, specific heat capacity, and viscosity of the NDG nanofluids, as well as their convective heat transfer behavior flowing in a double-pipe heat exchanger. To assess the thermal properties, we used various water-based nanofluids as coolants to analyze the total heat transfer coefficient, convective heat transfer coefficient, the percentage of wall temperature reduction, pressure drop, and pumping power in a counter-flow double-pipe heat exchanger. A novel MATLAB code carried out the calculations for Reynolds numbers between 5000 and 15,000 (turbulent flow) and nanosheet weight percentages between 0.00% and 0.06%. An increase in Reynolds number or the percentage of nanomaterial could perhaps enhance the heat transfer of the working fluid. As an example, using 0.06 wt.% nanomaterial in the base fluid led to 15.86% enhancement of the convective heat transfer coefficient in comparison with water. Nonetheless, the penalty in terms of the rise in the pumping power was rather small. For a particular material, increasing Reynolds number or nanomaterial weight percentage would augment pumping power. Power consumption, heat removal, and heat transfer rate were greater for nanofluids than for water in all investigated cases, for a particular pumping power. The average increase in heat transfer coefficient was nearly 16.2%. As a result, choosing NDG/water as the working fluid can improve the performance of double-pipe heat exchangers. [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]

In this paper, an analytical study is carried out on the heat transfer, pressure drop, and entropy generation in a flat-plate solar collector using Si0 2 lwater nanofluid with volume concentration of 1%. In the study, the effects of two different values of pH, i.e., 5.8 and 6.5, and two different sizes of nanoparticles, i.e., 12 nm and 16 nm, on the entropy generation rate in turbulent flow are investigated. The results are compared with the results obtained for the case of water. The findings show that by using the Brinkman model to calculate the viscosity instead of experimental data one obtains a higher heat transfer coefficient and thermal efficiency than that in the case of water, while, when the experimental data are used, the heat transfer coefficient and thermal efficiency of water are found to be higher than that of nanofluids. The results reveal that using nanofluids increases the outlet temperature and reduces the entropy generation rate. It is also found that for nanofluids containing the particles with a size of 16 nm, the increase in pH value would increase the entropy generation rate, while for nanoparticles with a size of 12 nm the increase in pH would decrease the entropy generation. [ABSTRACT FROM AUTHOR]

This paper is a continuation of the authors' previous work on the thermophysical properties, heat transfer, and pressure drop of nanofluids [Experimental Thermal and Fluid Science 52 (2014) 68–78]. In this paper, an experimental study is carried out to study the turbulent flow of COOH-functionalized multi-walled carbon nanotubes/water nanofluid flowing through a double tube heat exchanger. For this purpose, first, the thermophysical properties of the nanofluid, including the thermal conductivity and dynamic viscosity, have been measured at various temperatures and concentrations. Using the measured data, new correlations as a function of temperature and concentration are presented to predict the thermophysical properties. In the next step, the effects of low volume fractions of the nanofluid (from 0.05% to 1%) on the heat transfer rate are studied at the Reynolds numbers between 5000 and 27,000. The experimental results reveal that with increasing the nanofluid concentration, the heat transfer coefficient and thermal performance factor increase. On average, a 78% increase in heat transfer coefficient, a 36.5% increase in the average Nusselt number, and a 27.3% penalty in the pressure drop are recorded for the highest concentration of MWCNTs in water. [ABSTRACT FROM AUTHOR]

An experimental study has been performed to examine the effects of gap size on the flow pattern maps of air-water two-phase flow inside a mini-gap annular channel with inclination angles (θ) of 0°, 30°, and 60°. The tests are conducted for three different sizes of the inner diameters (Di) of the concentric annular test section-namely 8, 10, and 11 mm-while the outer diameter is equal to 12.5 mm. The flow pattern maps are presented for various types of flow including plug flow, slug flow, annular/slug flow, annular flow, bubbly/plug flow, bubbly/slug-plug flow, churn flow, and dispersed bubbly flow, regarding the alteration of gap sizes and inclination angles. According to the experimental conclusions, the angle of inclination and the various gap sizes are influential in the transition of flow regimes. [ABSTRACT FROM AUTHOR]

The heat transfer performance of various thermal devices may be augmented by active and passive techniques. One of the passive techniques is the addition of ultrafine particles (called nanoparticles) to the common heat transfer fluids so that the thermal transport properties of the prepared suspension (called nanofluid) will be enhanced as compared to the base fluid. Nanorefrigerants are a special type of nanofluids which are mixtures of nanoparticles and refrigerants and have a broad range of applications in diverse fields for instance refrigeration, air conditioning systems, and heat pumps. In this paper, a review is performed in order to clarify effect of nanorefrigerant properties (such as nanoparticle type, size and concentration) on heat transfer and pressure drop compared to pure refrigerant. Moreover, studies related to the thermophysical properties, flow and pool boiling, and applications of nanorefrigerants to some specific areas such as domestic refrigerators, heat pipes and air conditioners are also summarized. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an experimental study is performed to assess the heat transfer characteristics and pressure drop of low concentrations of a new class of nanotubes, i.e. COOH-functionalized double-walled carbon nanotubes (DWCNTs) suspended in water under turbulent flow in a double tube heat exchanger. First, the thermal conductivity and viscosity of nanofluids at volume fractions of 0.01%, 0.02%, 0.05%, 0.1%, 0.2%, and 0.4% are measured and corresponding correlations are presented. Next, the heat transfer and pressure drop of the nanofluids through the double tube heat exchanger are evaluated. The results indicate that even the use of low concentration of the nanofluid, i.e. 0.4%, leads to a remarkable increase in heat transfer coefficient (by 32%) in comparison with the distilled water. On the other hand, the pressure drop due to using the volume fraction of 0.4% raised by 20%. Finally, analysis of the heat transfer and pressure drop data via thermal performance factor reveals that in spite of the pressure drop penalty, the COOH-functionalized DWCNTs/water nanofluid with volume fraction of 0.4% is a good option to use in the double tube heat exchanger. [Copyright &y& Elsevier]

Abstract: In this investigation, a numerical model having two-dimensional equations was obtained by a CFD program and authors' experimental data were evaluated for the verification procedure of the numerical outputs. The experimental case study includes the single-phase flow of pure water in plain and micro-fin pipes whereas the numerical one has the simulated results of TiO2 particles suspended in single phase water flow in equivalent pipes at a constant heat flux. Hydrodynamics and thermal behaviors of the water–TiO2 flow were calculated by constant heat flux and temperature-dependent settings. Physical specifications of nanofluids were calculated by means of the results of authors' previous ANN analyses. This study illustrates local and average values of temperature, pressure, and velocity distributions in the tested pipes; furthermore, comparisons of pressure drop characteristics are given in terms of nanoparticle concentrations and tube types. [Copyright &y& Elsevier]