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We studied the effect of fin patterns on the air-side performance of a novel type of heat exchanger, namely louver spiral fin-and-tube heat exchangers (LSFTHXs). We investigated three types of fin patterns—radial, curved, and mixed-louver spiral fins—and compared their characteristics with plain spiral fin. We fabricated heat exchangers with a crimped aluminum fin base, two rows of steel tubes, and a fin pitch of 8.45 mm. All heat exchangers had both multipass parallel and counter cross-flow arrangements. The average heat transfer rate (Q ave), heat transfer coefficient (h o), and pressure drop (ΔP) increased along with frontal air velocity. The louver-spiral fin provided greater Q ave and h o than plain-spiral fin by about 9.7–15.6% and 13.4–27.1%, respectively. The j factors (Colburn factors) of louver-spiral fins were greater by approximately 10.4–13.1% (for mixed LSFTHX), 7.7–8.8% (for radial LSFTHX), and 2.1–5.1% (for curved LSFTHX) compared with the plain SFTHXs. Additionally, we found no significant difference in ΔP or f factor (friction factor) between louver-spiral and plain-spiral fins. LSFTHX configurations increased h o and j. [ABSTRACT FROM AUTHOR]

The novelty of this work is a new type of stack, namely, the parallel wave stack (PWS). In this paper, we study the performance of an air-based standing wave thermoacoustic refrigerator using the PWS. We use small heat pipes with a heat transfer rate of approximately 30 W as heat exchangers for the system. We investigate the effect of the ratio of stack length (Ls) to the distance between stack center and reflector (Xs) on the performance of the refrigerator parallel wave stacks with a blockage ratio (BR) of 67 %, 71 %, and 74 %, and with Ls/Xs ratios of 0.74, 0.96, and 1.20. We tested and compared these under a cooling load (Qc) of 0.5 W to 2 W and a frequency of 150 Hz. The results show that a BR of 71% with an Ls/Xs ratio of 0.96 provides a minimum cold-side temperature (Tc), and a maximum temperature difference (ΔTm), a maximum coefficient of performance, and a maximum relative coefficient of performance. This new type of stack can provide a higher performance. The structure of the stack is also stronger than others. This stack is an alternative and can be used in a thermoacoustic refrigerator. The results obtained from the present study will be very useful for improving the performance of the thermoacoustic refrigerator in the future. [ABSTRACT FROM AUTHOR]

This paper aims to summarize publications related to new generation of tube geommetry, thermal performance, and flow characteristics in different cross-sectional manners, and the effect of crucial parameters on thermal–hydraulic performance. An alternating cross-sectional tube has an enhanced vorticity without installing vortex generator devices. Multi-longitudinal vortices are induced along the length of the tube: this makes a significant enhancement in the thermal performance of the tube. Many published papers have focused on the heat transfer and flow characteristics of the alternating cross-section tubes in various configurations in the past two decades.Although the cross-sections had different shapes, the flow behaviors and vortex generation were similar. The geometric parameters, like aspect ratio, pitch length, transition length, alternating angle, phase-shifting, and the total length, influenced the thermal–hydraulic performance of the alternating cross-section tube. The flow characteristics in an annular region differed between the alternating cross-section tube as internal and external tubes. Moreover, some researchers proposed the Nu and f correlations for the practical design of heat exchangers. Finally, future research works and improvement guidelines for alternating cross-section tubes are revealed. [ABSTRACT FROM AUTHOR]

An investigation was experimentally performed to explore the influence of geometrical parameters on the evaporative heat transfer coefficient and pressure drop of R-134a in dimpled tubes. The test sections were the counter-flow tube-in-tube heat exchanger, where the refrigerant was evaporated in the inner tube and hot water flows in the annulus. Seven dimpled tubes with various helical pitches (p), dimpled pitches (z), and dimpled depths (e) were used as the test tubes. The test runs were carried out over mass fluxes of 300, 400, and 500 kg/m2s; heat fluxes of 20, 25, and 30 kW/m2; and evaporating temperatures of 7, 10, and 13 °C. The results show the heat transfer enhancement factor (HF) and pressure drop penalty factor (PF) increase with a rise in e and a decrease in p and z. The HF and PF range between 1.03–1.7 and 1.76–9.00, respectively. The correlations for predicting the Nusselt number and the friction factor of R-134a during evaporation in dimpled tubes are proposed. [ABSTRACT FROM AUTHOR]

The air side thermal performance of wavy fins with rectangular winglets are studied using experimental and numerical methods. The following parameters are selected for investigation: the attack angle, placed location, span and length of the rectangular winglet. The results show that all parameters affect the thermal performance of the air side. The best configurations of the winglet that provide the maximum j/f(1/3) for both winglet lengths are the 3.5 mm horizontal distance, 7 mm vertical distance, 60° attack angle and 1.25 mm winglet span. Moreover, the thermal performance comparisons between the optimal design of the wavy fin with winglet and normal wavy fin are reported. Wavy fins with winglets provide a larger heat transfer coefficient, pressure drop and j/f(1/3) than those of normal wavy fins. For the j/f(1/3) comparison, wavy fins with winglets provide a j/f(1/3) greater than that of normal wavy fins by 5.4–14.6 %, with the Reynolds number ranging from 1650 to 4423. [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 article presents two mathematical models for estimating the performances of finned tube heat exchangers with wavy configuration under partly and fully wet surface conditions. The actual dry bulb temperature method is the first one and the other is the method of equivalent dry bulb temperature. For the first one, the actual dry bulb temperature method that was firstly presented by our research team in 2015 is more uncomplicated than the well-known enthalpy potential method and is also easier than the equivalent dry bulb temperature method. Moreover, it has never been applicable to the finned tube heat exchanger. For accuracy, the finned tube heat exchangers are distinguished into teeny segments and every segments are separated by conditions of surface into three groups: partially wet segments, fully wet segments, fully dry segments. This procedure is called the finite circular fin method that was firstly presented by our research in 2007. From the results, the heat and mass transfer characteristics estimated by the method of equivalent dry bulb temperature are greater than those estimated by the method of actual dry bulb temperature. This is the influence of the non-constant terms in both methods. Moreover, the correlations for the conditions of party and fully wet surface are revealed for predicting the heat and mass transfer coefficients of the finned tube heat exchanger with wavy configuration. The users can apply our presented models with the proposed correlation for predicting the heat and mass transfer performances of finned tube heat exchangers with wavy configuration under the conditions of fully and partly wet surface. [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]

This paper is a continuation of the authors’ previous work. Theoretical and experimental studies of the heat transfer characteristics of a helical-coiled, crimped, spiral, finned-tube heat exchanger in dry surface conditions are presented. The test section is a helical-coiled, finned-tube heat exchanger. The coil unit is composed of four concentric helical-coiled tubes of different diameters. All tubes are constructed by bending straight copper tube into seven layers of helical coil. Aluminum crimped spiral fins, with an outer diameter of 28.25 mm and a thickness of 0.5 mm, are connected around the tube. Hot water is used as a working fluid for the tube side, while ambient air is used for the shell side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. A mathematical model is developed and the simulation results show reasonable agreement with the experimental data. [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]

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: This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46mm. Similarly, the corrugation depths are 1, 1.25, and 1.5mm, respectively. The test conditions are performed at saturation temperatures of 40–50°C, heat fluxes of 5–10kW/m2, mass fluxes of 200–700kg/m2 s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes. [Copyright &y& Elsevier]

Abstract: The effects of corrugation pitch on the evaporation of R-134a flowing inside horizontal corrugated tubes are investigated. The test section is a tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water flowing in the annulus. Smooth and corrugated tubes having similar inside diameters are used as the inner tube. Three corrugated tubes having a corrugation depth fixed at 1.5mm and corrugation pitches of 5.08, 6.35 and 8.46mm are examined. The data obtained from smooth tube are plotted and compared with the flow pattern map established by and . The effects of average vapour quality, equivalent Reynolds number and corrugation pitch are discussed. The maximum ratios of heat transfer enhancement factor to pressure drop penalty factor (Nuc/Nus)/((ΔP/L)c/(ΔP/L)s) is approximately 1.2. [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: This study presents the airside performance of the fin-and-tube heat exchangers having plain fin geometry with a larger diameter tube (Dc =15.88mm) under dehumidifying condition. A total of nine samples of heat exchangers subject to change of the number of tube row and fin pitch are made and tested. It is found that the effect of fin pitch on the sensible j factor is, in general, diminished with the rise of tube row. However, there is a unique characteristic of fin pitch at a shallow tube row, the heat transfer performance is first increased at a wider pitch but a further increase of fin pitch lead to a falloff of heat transfer performance due to interactions amid flow development and bypass flow. The influence of tube row on the airside performance is rather small for both heat transfer and frictional characteristics at a fin pitch of 2.1mm and when the Reynolds number is less than 4000. A slight deviation of this effect is encountered when fin pitch is increased to 2.54mm or 3.1mm due to condensate adhered phenomena. [Copyright &y& Elsevier]

This article concerns the pressure drop caused by using the electrohydrodynamic (EHD) technique during evaporation of pure R-134a inside smooth and micro-fin tubes. The test section is a counter-flow concentric tube-in-tube heat exchanger where R-134a flows inside the inner tube and hot water flows in the annulus. A smooth tube and micro-fin tube having an inner diameter of 8.12 mm and 8.92 mm, respectively, are used as an inner tube. The length of the inner tube is 2.50 m. The outer tube is a smooth copper tube having an inner diameter of 21.2 mm. The electrode, which is a cylindrical stainless steel wire having diameter of 1.47 mm, is placed in the center of the inner tube. The electrical field is established by connecting a DC high voltage power supply of 2.5 kV to the electrode while the inner tube is grounded. Experiments are conducted at saturation temperatures of 10-20°C, mass fluxes of 200-600 kg/m2s, and heat fluxes of 10-20 kW/m2. The experimental results indicate that the application of EHD introduces a small pressure drop penalty. New correlations for the pressure drop are proposed for practical applications. [ABSTRACT FROM AUTHOR]

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: The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail. [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 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]

Abstract: The present study proposes a new reduction method to calculate the heat and mass transfer characteristics of the wavy fin-and-tube heat exchangers under dehumidifying conditions. For fully wet conditions, the sensible heat transfer and mass transfer characteristics are relatively insensitive to the inlet relative humidity. The heat and mass transfer performances show appreciable influence of fin spacing at 1-row configuration. Both the heat and mass transfer performances increase when the fin spacing is reduced. However, the difference becomes less noticeable when Re Dc >3000. For 1-row configuration, larger wave height shows much larger difference with the fin spacing. However, the effect of inlet conditions and geometrical parameters on the heat and mass performance becomes less significant with the rise of number of tube rows. Test results show that the heat and mass transfer analogy is roughly applicable (the ratios of h c,o/h d,o C p,a are in the range 0.6–1.1, and is insensitive to change of fin spacing). The correlations are proposed to describe the heat and mass transfer characteristics. These correlations can describe 94.19% of the j h factors within 15% and 83.72% of the j m factors within 15%. Correspondingly, 93.02% of the ratios of h c,o/h d,o C p,a are predicted by the proposed correlation within 15%. [Copyright &y& Elsevier]

In the present study, the experimental and theoretical results of the heat transfer characteristics and performance of a spirally coiled, finned-tube heat exchanger under sensible cooling conditions are investigated. The test section is a spiral-coil heat exchanger, which consists of a steel shell and a spirally coiled finned-tube unit. The spiral-coil unit consists of six layers of concentric, spirally coiled, finned tubes. Each tube is fabricated by bending a 9.6 mm-diameter straight copper tube into a spiral-coil of four turns. The innermost and outermost diameters of each spiral-coil are 145.0 and 350.4 mm, respectively. Aluminium annular fins with a thickness of 0.6 mm and outer diameter of 28.4 mm are placed helically around the tube. The edge of each fin at its inner diameter is corrugated. Air and water are used as working fluids in the shell side and tube side, respectively. A mathematical model based on mass and energy conservation is developed to simulate the heat transfer characteristics and performance of the heat exchanger. The results obtained from the present model show reasonable agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Abstract: An experimental study is conducted to investigate the effects of a fin pitch and number of tube rows on the air side performance of fin and tube heat exchangers having herringbone wavy fin configuration at various fin thicknesses. A total of 10 samples of fin and tube heat exchanger with a tube outside diameter of 9.53mm, transverse tube pitch of 25.4mm and longitudinal tube pitch of 19.05mm, having various fin pitches, number of tube rows and fin thicknesses, are tested in a well insulated open wind tunnel. The heat exchangers are made from aluminium plate finned, copper tube. Ambient air is used as a working fluid in the air side while hot water is used for the tube side. The results are presented as the variation of the Colburn factor and the friction factor with the Reynolds number based on the fin collar outside diameter (). The experimental results reveal that the fin pitch has an insignificant effect on the heat transfer characteristic. The friction factor increases with increasing fin pitch when >2500, approximately. The Colburn factor and the friction factor decrease with increasing number of tube rows when <4000, approximately. These results remain the same when the fin thickness is changed. [Copyright &y& Elsevier]

Abstract: The heat transfer characteristics and the performance of a spiral coil heat exchanger under cooling and dehumidifying conditions are investigated. The heat exchanger consists of a steel shell and a spirally coiled tube unit. The spiral-coil unit consists of six layers of concentric spirally coiled tubes. Each tube is fabricated by bending a 9.27mm diameter straight copper tube into a spiral-coil of five turns. Air and water are used as working fluids. The chilled water entering the outermost turn flows along the spirally coiled tube, and flows out at the innermost turn. The hot air enters the heat exchanger at the center of the shell and flows radially across spiral tubes to the periphery. A mathematical model based on mass and energy conservation is developed and solved by using the Newton–Raphson iterative method to determine the heat transfer characteristics. The results obtained from the model are in reasonable agreement with the present experimental data. The effects of various inlet conditions of working fluids flowing through the spiral coil heat exchanger are discussed. [Copyright &y& Elsevier]

Abstract: In the present study, the average tube-side and air-side heat transfer coefficients in a spirally coiled finned tube heat exchanger under dry- and wet-surface conditions are experimentally investigated. The test section is a spiral-coil heat exchanger, which consists of six layers of concentric spirally coiled tube. Each tube is fabricated by bending a 9.6-mm outside diameter straight copper tube into a spiral coil of four turns. Aluminium fins with thickness 0.6 mm and outside diameter 28.4 mm are placed helically around the tube. The chilled water and the hot air are used as working fluids. The test runs are done at the air and water mass flow rates ranging between 0.02 and 0.2 kg/s and between 0.04 and 0.25 kg/s, respectively. The inlet-air and -water temperatures are between 35 and 60 °C and between 10 and 35 °C, respectively. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. New correlations based on the data gathered during this work for predicting the tube-side and air-side heat transfer coefficients for the spirally coiled finned tube heat exchanger are proposed. [Copyright &y& Elsevier]

An introduction is presented in which the editor discusses various reports within the issue on topics including the thermal design of plate fin heat exchangers, studies on dropwise condensation, and the modification of Wilson Plots.

Heat exchanger is a basic component used in thermal processes such as industrial fluid heating, household appliances, waste heat recovery units (WHRU), etc. Presently, the spiral fin-and-tube heat exchanger is used as a favored type of heat exchanger for the waste heat recovery unit system, i.e., the economizer heat exchanger. The study investigates the effect of fin pitches on the air-side performance of serrated welded spiral fin-and-tube heat exchangers having Z-shape flow arrangement and the number of tube rows of 2 in range of high Reynolds numbers (4000–15,000). The test samples are made from steel (iron) with different fin pitches of 3.6, 4.2, and 6.2 mm. The experimental results reveal that the fin pitch has a significant effect on the air-side performance. It is confirmed that the fin pitch of 3.6–6.2 mm has the same trend in heat transfer coefficient at the tested condition. For f -friction factor, a detectable rise of the friction factor is found when the fin pitches increase to 6.2 mm. Moreover, the air-side performance correlations of serrated welded spiral finned tube heat exchangers are proposed for practical industrial applications. The proposed air-side performance correlation describes 100% for j -Colburn factor and f -friction factor within 10%. [ABSTRACT FROM AUTHOR]

In the present study, an attempt has been made to summarize and analyze the results of an examination of the air-side performance of spiral (or helical) fin-and-tube heat exchangers. Currently, the spiral fin-and-tube heat exchanger is a favored type of heat exchanger for the waste heat recovery unit (WHRU), a kind of economizer system. The present paper is broadly divided into an experimental section and numerical and simulation sections. A significant fraction of the papers herein reviewed pertains to the effect of fin configurations, tube arrangements, operating conditions, and other factors on the air-side performance of the spiral fin-and-tube heat exchangers. Approximately 40 published articles related to spiral fin-and-tube heat exchangers are briefly described. Moreover, the air-side performance correlations of spiral fin and circular fin-and-tube heat exchangers are compiled into this work for practical industrial applications. [ABSTRACT FROM AUTHOR]

• Condensation flow of R-134a and R-513A in a horizontal smooth tube is studied. • Condensation flow of R-513A in horizontal smooth and micro-fin tubes is studied. • R-513A can replace R-134a with a lower heat transfer coefficient and pressure drop. • The micro-fin tube can enhance the heat transfer coefficient by 69 %. • The enhancement in pressure drop for a micro-fin tube is up to 120 %. An experimental investigation to determine the average heat transfer coefficient (h av) and frictional pressure drop (ΔP F) characteristic of R-513A and R-134a in a smooth tube (ST) and micro-fin tube (MFT) is carried out. A 2500 mm long counter-flow tube-in-tube heat exchanger made of copper is used for conducting experiments. The ST and MFT with inner diameters (D i) of 6.74 and 6.82 mm, respectively, are experimentally tested for various operating conditions such as saturation temperatures (T sat = 40 to 50 °C), heat fluxes (q" =9.8 to 19.6 kW/m2), mass fluxes (G = 575 to 770 kg/m2s) and average vapor qualities (x av = 0.2 to 0.9). The influence of q", G, and T sat on h av and ΔP F are discussed. First, a comparison between R-513A and R-134a in an ST showed maximum reductions of 16 and 20 % in h av and ΔP F, respectively for R-513A, when compared to R-134a. Second, the MFT, due to its higher heat transfer area, resulted in 69 % and 120 % enhancements in h av and ΔP F , respectively, compared to that of an ST. Hence, the findings of this research provide information that is currently useful to the thermal industries, affirming that R-513A can be a suitable long-term alternative refrigerant. [ABSTRACT FROM AUTHOR]

Abstract: This study conducted experiments on the optimized fin pitch for crimped spiral fin-and-tube heat exchangers. The experiments covered a size range of 2.4–6.5mm, which is the manufacturing limitation for this kind of fin. The water-flow arrangement used in this experiment combined the parallel cross-flow and the counter cross-flow in a two-row configuration. Ambient air was used as the working fluid on the air-side, and hot water was used on the tube-side. The effects of fin pitches on the heat transfer coefficient and pressure drop characteristics were studied. The results clearly showed that the convective heat transfer coefficient (ho ) for a fin pitch of 2.4mm is relatively low compared with that of other fin pitches with the same air frontal velocity. Using larger fin pitches (i.e., 4.2, 6.2, and 6.5mm) resulted in negligible differences in the pressure drop. Moreover, this work introduces the parameter of three performances indexes, which can be expressed as the ratio of the desired output to the required input, for optimization purposes. Due to the difference in optimum fin pitch obtained by these performance indexes, an intersection analysis was conducted. The results indicated that the optimum fin pitch is 4.2mm for this work, which could be valuable for the effective design for industrial thermal-system applications. [Copyright &y& Elsevier]

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: Correlations are proposed for the wet surface ratio of a fin-and-tube heat exchanger with plain and wavy fin geometry under dehumidifying conditions. The ‘Finite Circular Fin Method’ (FCFM) is used for data reduction. It is found that the percentage of wet surface area increases with increasing fin spacing or number of tube rows and decreasing Reynolds number. Despite the addition of tube rows or reduced fin spacing the effective surface area is increased, and its influence on a partially wet surface is apparently the opposite. This is because adding tube rows will provide a more effective temperature drop in air flow than adding fins, Moreover, the heat and mass transfer characteristics of jh and jm increase with an increase in the area of wet surface. Correlations for prediction of the percentage of wet surface area are proposed. These correlations can describe 83.81% of the area of wet surface to within ±10%. [Copyright &y& Elsevier]

Heat transfer enhancement using the electrohydrodynamic (EHD) technique during evaporation inside horizontal smooth and micro-fin tubes is experimentally investigated. The test section is a counter-flow concentric tube-in-tube heat exchanger with R-134a flowing inside inner tube and heating water flowing in the annulus. Smooth and micro-fin inner tubes are used as test section. The outer diameter and length of inner tubes are 9.52mm and 2.50m, respectively. The outer tube is a smooth copper tube having inner diameter of 27mm. The electrode is made from stainless steel rod of 1.47mm diameter. Experiments are performed at average saturation temperatures ranging between 10 and 20°C, heat fluxes ranging between 10 and 20kW/m2 at mass flux of 400kg/m2 s and applied voltage of 2.5kV. The maximum heat transfer enhancement ratios are 1.25 and 1.15 for smooth tube and micro-fin tube, respectively. New correlations based on the data gathered during this work for predicting the evaporation heat transfer coefficient in smooth tube and micro-fin tube are proposed for practical applications. [Copyright &y& Elsevier]

Abstract: This paper presents a review of data reduction method for heat and mass transfer characteristics of fin-and-tube heat exchangers with dehumidification. There are many reduction methods for fin-and-tube heat exchangers under dehumidifying conditions. The data reduction methods being reviewed includes the original Threlkeld method, direct method, the equivalent dry-bulb method, tube by tube method, the fully wet and fully dry tiny circular fin method, and the finite circular fin method. Among these methods, the original Threlkeld method, direct method, the equivalent dry-bulb method are lumped method while others can divide the fin-and-tube heat exchangers into small segments for more accuracy in handling the surfaces to be fully dry, fully wet, or partially wet. In addition, the mass transfer characteristics can be obtained from the modified process line equation incorporated with the preceding methods. It should be noted that the conventional assumption of constant ratio (hc,o /hd,oCp,a ≈constant) is actually incorrect. This present paper can be used as the first guideline for the researcher for reducing the experimental data for fin-and-tube heat exchangers under dehumidifying conditions. [Copyright &y& Elsevier]

Abstract: The present study proposes the finite circular fin method for analyzing the heat and mass transfer characteristics of wavy fin-and-tube heat exchangers under fully and partially wet surface conditions. The analysis is carried out by dividing the wavy fin-and-tube heat exchanger into many tiny segments. The tiny segments can be analyzed based on surface conditions, i.e. fully wet, fully dry or partially wet surface condition. From the experimental results, it is found that the heat and mass transfer characteristics are insensitive to the inlet relative humidity but the effect of relative humidity on mass transfer characteristic become more pronounced when the partially wet surface condition takes place. The heat transfer characteristic is independent of the fin spacing. Effect of fin spacing on mass transfer characteristic is small when fin spacing is larger than 2.5mm. However, at smaller fin spacing, the mass transfer characteristic slightly decreases when the relative humidity increases. The ratios of h c,o/h d,o C p,a are in the range of 0.6–1.2. Correlations are proposed to describe the heat and mass transfer characteristics. These correlations can describe 95.63% of the heat transfer characteristic within 15% and 95.14% of the mass transfer characteristic within 20%. Correspondingly, 94.68% of the ratios of h c,o/h d,o C p,a are predicted by the proposed correlation within 20%. [Copyright &y& Elsevier]

Abstract: The heat transfer duty of heat exchangers can be improved 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. Electrohydrodynamic (EHD) techniques have been introduced as one of the types of active heat transfer enhancement techniques. This paper presents a review of research works on electrohydrodynamic heat transfer enhancement. This paper can be used as the first guideline for the researcher in using EHD techniques for heat transfer enhancement. [Copyright &y& Elsevier]

Abstract: This study investigated the effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions. A total of 10 samples were tested with associated fin thickness (δ f) of 0.115mm and 0.25mm, respectively. For a heat exchanger with two rows (N =2) and fin pitch F p of 1.41mm, the effect of fin thickness on the heat transfer coefficient is more pronounced. The heat transfer coefficients for δ f =0.25mm is about 5–50% higher than those for δ f =0.115mm whereas the pressure drop for δ f =0.25mm is about 5–20% higher. The unexpected difference in heat transfer coefficient subject to fin thickness is attributable to better interactions between the directed main flow and the swirled flow caused by the condensate droplet for δ f =0.25mm. The maximum difference in heat transfer coefficients for N =2 and F p =2.54mm subject to the influence of fin thickness is reduced to about 20%, and there is no difference in heat transfer coefficient when the frontal velocity is above 3m/s. For N ⩾4 and F p =2.54mm, the influence of fin thickness on the heat transfer coefficients diminishes considerably. This is because of the presence of tube row, and the unsteady/vortex shedding feature at the down stream of wavy channel. Based on the present test results, a correlation is proposed to describe the air-side performance for wavy fin configurations, the mean deviations of the proposed heat transfer and friction correlations are 7.9% and 7.7%, respectively. [Copyright &y& Elsevier]

• Simulation of nanofluid in a heat exchanger equipped with twisted tape using a two-phase mixture model. • Studying effect of nanofluid and twisted tape on the thermal performance of heat exchanger. • Evaluating effect of material type of twisted tape on heat transfer and pressure drop in the heat exchanger. • The use of twisted tape improved the Nusselt number up to 22%. • Alumina nanoparticles increased heat transfer and pressure drop up to 30% and 40%, respectively. The present study dealt with the numerical simulation of nanofluid flow in a double tube heat exchanger equipped with twisted tape. Alumina/water nanofluid and pure water are considered to be working fluids. A two-phase mixture model was employed for nanofluid flow simulation. The effect of nanofluid and twisted tape on the hydrodynamic and thermal performance of the heat exchanger was studied. Next, the focus was placed on the effect of the surface roughness (material type) of twisted tape on heat transfer and pressure drop in the heat exchanger. The results are presented as the temperature distribution, velocity field, Nusselt number, and pressure drop for various Reynolds numbers, nanofluid concentrations, pitch ratios, and tape insert materials. The results revealed that the use of twisted tape improved the Nusselt number up to 22%; also, adding alumina particles to water augmented heat transfer up to 30% and increased pressure drop up to 40%. Tapes with more roughness provided heat transfer enhancement of up to 16%, whereas the friction factor increased up to 21%. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Researchers in heat transfer field always attempt to find new solutions to optimize the performance of energy devices through heat transfer enhancement. Among various methods which are implemented to reinforce the thermal performance of energy systems, one is the dispersion of solid nanoparticles in common working fluids such as water. The suspension is called nanofluid. On the other hand, utilizing porous media in heat exchangers is another technique to augment of thermal efficiency. Porous media by providing high surface area contact will ameliorate heat transfer rate in ducts. In the present work, a comprehensive review is conducted on the simultaneous application of nanofluids and porous media for heat transfer enhancement purposes in thermal systems with different structures, flow regimes, and boundary conditions. [ABSTRACT FROM AUTHOR]

• Novel heat exchanger made by Alternating cross-section flattened tube is proposed. • Thermal performance of the heat exchanger is the first experimentally studied. • The effects of main parameters on thermal performance are discussed. • Alternating cross-section flattened tube heat exchanger has a dominant performance. The air-side performance of a novel alternating cross-section flattened (ACF) tube heat exchanger was experimentally investigated. Three heat exchangers with an in-line tube arrangement and two tube rows were used as the test sections. The tube configuration is the highlight of the research work. ACF tubes with hydraulic diameters of 4.16, 4.75, and 5.20 mm were assembled as the heat exchanger. The experimental results showed that the ACF tube heat exchangers exhibited a prominent heat transfer characteristic with a reasonable pressure loss penalty. Performance evaluation criteria, as a nondimensional parameter, were employed to assess the heat exchanger performance. Finally, the j and f of the ACF tube heat exchanger were compared with those obtained from various shapes of noncircular tube and fin tube heat exchangers. All the details are examined and discussed in this study. [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]

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 study, the condensation heat transfer characteristics of R-134a flowing inside multiport minichannels were investigated. The multiport minichannel-tested tubes have 14 channels with a 1.1mm hydraulic diameter, and eight channels with a 1.2mm hydraulic diameter were designed as a counterflow tube-in-tube heat exchanger. The experiment was performed with mass fluxes of refrigerant between 340 and 680kg/m2s, with 15, 20, and 25kW/m2 heat fluxes, and saturation temperatures of 35–45°C. The flow pattern for the experimental data was initially predicted using existing flow pattern maps. It could be noted that the annular flow pattern existed for most of the experimental data. Results showed that the average heat transfer coefficient increased with the increase of vapor quality, mass flux, and heat flux, but decreased as saturation temperature rose. When compared with two correlations obtained from condensation inside the multiport minichannels, the heat transfer coefficient could be predicted within an acceptable range. [Copyright &y& Elsevier]

Abstract: The purpose of this experimental investigation is to investigate the effects of fin pitch (i.e., fp of 2.4, 3.2, and 4.2mm) on the air-side heat transfer performance and frictional characteristics of L-footed spiral fin-and-tube heat exchangers at high Reynolds numbers (Redc ) of 4000–15,000. A determinant of the parallel cross-flow and the counter cross-flow is the flow arrangement of the test heat exchangers. Ambient air and hot water are used as a working fluid on the air- and the tube-side, respectively. The results indicate that the air-side heat transfer coefficient and Colburn factor are independent of fin pitch. However, fin pitch does have an influential effect on the average heat transfer rate, pressure drop, and friction factor. In terms of industrial applications, the correlation of the Colburn factor and friction factor are proposed for practical applications. [Copyright &y& Elsevier]

This study proposes a new method, namely the "fully wet and fully dry tiny circular fin method," for analyzing the heat and mass transfer characteristics of plain fin-and-tube heat exchangers under dehumidifying conditions. The present method is developed from the tube-by-tube method proposed in the previous study by the same authors. The analysis of the fin-and-tube heat exchangers is carried out by dividing the heat exchanger into many tiny segments. A tiny segment will be assumed with fully wet or fully dry conditions. This method is capable of handling the plain fin-and-tube heat exchanger under fully wet and partially wet conditions. The heat and mass transfer characteristics are presented in dimensionless terms. The ratio of the heat transfer characteristic to mass transfer characteristic is also studied. Based on the reduced results, it is found that the heat transfer and mass transfer characteristics are insensitive to changes in fin spacing. The influence of the inlet relative humidity on the heat transfer characteristic is rather small. For one and two row configurations, a considerable increase of the mass transfer characteristic is encountered when partially wet conditions take place. The heat transfer characteristic is about the same in fully wet and partially wet conditions provided that the number of tube rows is equal to or greater than four. Correlations are proposed to describe the heat and mass characteristics for the present plain fin configuration. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the effect of fin pitches and fin materials on the air-side performance of crimped fin-and-tube heat exchangers in the range of high Reynolds numbers (4000–13000). The test samples are made from copper and aluminium with different fin pitches (f p =3.2,4.2 and 6.2mm). It is found that the proposed simple average effectiveness equation from the pure counter and parallel circuitry arrangement can well represent the effectiveness-NTU relationship for the current z-shape arrangement. The experimental results reveal that the fin pitch casts insignificant effect on the heat transfer characteristics (Colburn j factor). However, a detectable rise of the friction factor is seen when the fin pitch is increased to f p =6.2mm. On the other hand, the effect of fin material on the airside performance is negligible. [Copyright &y& Elsevier]

• The crimped spiral fin for small diameter tube is experimentally investigated. • Effects of fin pitch and fin outside diameter on air-side performance are studied. • The fin pitch and fin outside diameter are important key of friction characteristic. • New correlations of the Colburn factor and friction factor are proposed. The performance of crimped spiral fin-and-tube heat exchangers (CSFTHXs) with a small tube diameter at a Reynolds number of 1500-6400 based on the tube's outer diameter is experimentally investigated. An important factor in this study is the small-diameter tubing, with an outer diameter of 9.53 mm. Copper tubes and aluminum fins are assembled as CSFTHXs. Test sections consist of a fin thickness of 0.5 mm, two tube rows, and various fin pitches and outer diameters of 3.18-6.35 mm and 23.0-28.5 mm, respectively. Hot water and fresh air are used as the working fluids. The effects of fin pitch and fin outer diameter on the heat transfer characteristics and friction factor (f) are also studied. The results show that the fin's pitch and outer diameter have little effect on the Nusselt number (Nu) and Colburn factor (j). On the other hand, they have a significant effect on the f. Furthermore, correlations between the Nu, j , and f for a CSFTHX with a small tube diameter are developed and proposed for designing heat exchangers in the thermal industry. [ABSTRACT FROM AUTHOR]

Due to their superior thermophysical properties, there is a growing body of work on nanofluids in the field of thermal systems. However, there is no specific review of the role of the nanoparticle shape, which has been found crucial to their performance adjustment. A comprehensive literature review of the effect of nanoparticle shape on the hydrothermal performance of thermal systems utilizing nanofluids was compiled. The review covered the forced, mixed, and natural convection regimes and included heat exchangers, boundary layer flows, channel flows, peristaltic flows, impinging jets, cavity flows, and flows of hybrid nanofluids. It indicated that the control of nanoparticle shape is a promising technique for the optimization of heat exchange and the required pumping power. However, no uniform conclusion was reached for the role of nanoparticle shape on the hydrothermal performance of thermal systems. In most of the previous studies in the natural and forced convection regimes, the platelet–like nanoparticle acquired the highest heat transfer rate. However, most of the works in the mixed convection regime reported the best heat transfer performance for the blade–like nanoparticle. More research studies are required in future to determine the role of nanoparticle shape for thermal management of energy systems. • Nanoparticle shape is crucial to the performance control of various nanofluids. • The effects of nanoparticle shape on thermal system performance were reviewed. • Nanoparticle shape control is a promising technique for heat transfer optimization. • More research is required for the role of nanoparticle shape in thermal systems. [ABSTRACT FROM AUTHOR]