Heat Transfer Characteristics Of R134a Research Articles

Two-phase flow condensation heat transfer characteristics of R-134a inside three-dimensional cylindrical micropillar enhanced tube

This paper presents an experimental investigation for evaluation of heat transfer coefficient, frictional pressure drops and flow regimes with their transitions during the condensation of R-134a inside a smooth, two microfins (MF1 and MF2) and a newly developed micropillar (MP) tubes. The experiments are carried out for a range of mass flux at an average saturation temperature of 35°C. Micropillar tube has produced the maximum heat transfer coefficient, while the microfin (MF1) tube resulted the highest frictional pressure gradient compared to other tubes. The experimental observations of microfin and micropillar tubes are plotted on the mass flux versus vapor quality flow map and Taitel and Duckler flow map to discuss the transitions within different flow patterns achieved. Visual inspection of the flow regimes with varying mass flux and vapor quality showed stratified-wavy, intermittent, and annular flow regimes. The occurrence of annular flow is found to be more in MF1 and MP tube. A performance evaluation factor (PEF) is defined to evaluate the thermal-hydraulic performance of the tubes. Using the same, it has been observed that the newly developed MP tube has a PEF value greater than unity which suggest augmentation of heat transfer coefficient at the expense of lesser pressure drop penalty.

Enhancement of Cooling Capacity of Refrigerant R-134a through the Addition of CuO and Silica Nanoparticles

Abstract: This study investigates the cooling capacity enhancement of refrigerant R-134a by dispersing copper oxide (CuO) and silica (SiO2) nanoparticles in the base fluid. The nanoparticles were mixed at different concentrations, and their effects on the thermophysical properties and heat transfer characteristics of R-134a were evaluated. Experimental results demonstrated that the addition of CuO and SiO2 nanoparticles significantly improved the cooling capacity of R-134a, highlighting the potential of nanofluids in enhancing refrigeration systems. The demand for efficient and environmentally friendly refrigeration systems has led to extensive research in the field of nanofluids. This study investigates the cooling capacity enhancement of refrigerant R-134a by dispersing copper oxide (CuO) and silica (SiO2) nanoparticles in the base fluid. The nanoparticles were mixed at different concentrations, and their effects on the thermophysical properties and heat transfer characteristics of R-134a were evaluated. Experimental results demonstrated that the addition of CuO and SiO2 nanoparticles significantly improved the cooling capacity of R-134a, highlighting the potential of nanofluids in enhancing refrigeration systems.

Numerical simulation of condensation heat transfer characteristics of R134a at high vapor velocity under different angles

Heat pipe is widely used as an efficient built-in heat exchanger for thermal management of high pulse and heat flux power supplies. Enhanced heat dissipation at the condensing side is an important issue in heat pipe research, where vapor velocity is an essential factor affecting the condensing heat transfer characteristics. This study adopted the CFD method to numerically simulate the condensation heat transfer of R134a flowing in a smooth pipe. The flow condensation heat transfer characteristics, such as film thickness and heat transfer coefficient (HTC), were investigated under vapor velocities v = 20–100 m/s and tube inclinations β =-90–90° Geometric and numerical heat transfer modeling is established in high vapor velocity flow condensation. The error of numerical simulation results and model validation is 10%. The HTC increases rapidly with the change in vapor velocity under 60 m/s, increasing the rate to 0.4. When the vapor velocity exceeds 60 m/s, the growth trend slows to 0.15, 37.5% of the original increase. At high vapor velocities above 60 m/s, the effect of pipe inclination on condensation heat transfer coefficient can be almost negligible. A modified empirical correlation suitable for high vapor velocities is proposed, which has higher predictive accuracy when the vapor velocity exceeds 80 m/s.

A Numerical Study on Condensation Heat Transfer Characteristics of R134a in Microchannel Under Varying Gravity Conditions

A Numerical Study on Condensation Heat Transfer Characteristics of R134a in Microchannel Under Varying Gravity Conditions

Liquid film boiling on plain and structured tubular surfaces with and without hydrophobic coating

In this study, thin liquid film boiling heat transfer characteristics of R134a on a plain and two finned tubular surfaces were experimentally investigated. The dependence of heat transfer coefficient (HTC) on the composite effects of microstructure and hydrophobic coating was characterized. It is found that HTC increases monotonously with increasing heat flux until reaching a threshold heat flux (THF), beyond which the HTC starts to descend. The THF is highest for the plain surface and lowest for the boiling-enhanced surface due to the effect of reentrant cavities. The boiling-enhanced surface shows larger HTC under low heat fluxes (less than 60 kW m−2) compared with the condensation-enhanced counterpart, whereas the latter one is superior when the heat flux exceeds 60 kW m−2. Hydrophobic coating can substantially intensify the liquid film boiling heat transfer on boiling-enhanced finned surface with reentrant cavities. The intensification of heat transfer is more prominent in cases of high heat flux.

Experimental study on film condensation heat transfer characteristics of R134a, R1234ze(E) and R1233zd(E) over condensation tube with enhanced surfaces

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

An Experimental Study of R134a Condensation Heat Transfer in Horizontal Smooth and Enhanced Tubes

Abstract In this paper, the condensation heat transfer characteristics of R134a inside enhanced tubes using two types of surface structures with different materials were investigated, which were then compared with plain tubes under the same test conditions. The enhanced tubes were: 1EHTa tube with dimpled and petal arrays structure and 1EHTb tube with protrusion and similar petal arrays structure. The experiment was conducted for a mass flux ranging from 100 to 200 kg m−2 s−1 with saturation temperature of 318.15 K. The inlet and outlet vapor qualities were fixed at 0.8 and 0.2, respectively. The test tubes had the same outer diameter of 12.7 mm. Results showed that the dimpled and protruded surface tubes enhanced the convection condensation heat transfer and the heat transfer coefficient was 1.4–1.6 times higher than that of the smooth tube. Heat transfer enhancement of the 1EHTa and 1EHTb tube was mainly due to the complex roughness surface structures that created swirling and increased the interface turbulence. The condensation heat transfer coefficient increased slightly with increasing mass flux. The pressure drop penalty was found to increase as mass flux increased. Compared with the smooth tube, the pressure drop of Cu-1EHTa tube, SS-1EHTa tube, and Cu-1EHTb tube were 1.15, 1.21, and 1.14 of smooth tube, respectively. Enhanced tubes exhibited higher performance factors (PFs) compared to the smooth tube. The average PF was 1.3–1.5. A new correlation of heat transfer coefficient has been developed within ±15% error band.

Experimental study on flow boiling heat transfer characteristics of R134a, R245fa and R134a/R245fa mixture at high saturation temperatures

The flow boiling heat transfer characteristics of R245fa, R134a and non-azeotropic mixture R134a/R245fa (0.33/0.67 by mass) at high saturation temperatures were investigated in a horizontal tube with 10 mm I.D. Effects of high saturation temperatures on heat transfer coefficients of R134a, R245fa and the mixture were investigated from 55 °C to 95 °C. Heat flux was controlled from 6 to 24 kW/m2 and mass flux ranged from 100 to 300 kg/m2.s. The results show the heat transfer coefficient of the mixture (R134a/R245fa with 33%/67% by mass) is much lower than that of pure R134a and is close to that of pure R245fa. At high saturation temperatures, mass flow rate has less influence on heat transfer coefficient of R245fa and the mixture at a large heat flux. The heat transfer coefficients of three working fluids increase slightly when the saturated temperatures change within 10 °C. The pressure drops of pure R245fa and the binary mixture were studied and the pressure drop of the mixture (R134a/R245fa with 33%/67% by mass) is a little higher than that of R245fa.A modified heat transfer correlation considering the mixing effect of binary mixture is proposed for the mixture of R245fa/R134a, and the mean absolute deviation is within 20%. A modified two phase frictional factor for the mixture is proposed and the deviation of the mixture using modified two phase frictional factor is between −6% and +22%.

Enhanced flow boiling heat transfer characteristics of R134a on graphene-Cu nanocomposite coating on copper substrate

This study presents the flow boiling heat transfer characteristics of graphene-Cu nanocomposite coated copper surfaces. First, recently reported studies regarding efforts to enhance flow boiling heat transfer are discussed, then related studies on the impacts of using nanocomposite coated surfaces on flow boiling heat transfer characteristics are reviewed. Experimental investigations explore the effects of graphene concentration and feed mass flux of a refrigerant (R134a) on the heat flux and the heat transfer coefficient. The test chamber is a rectangular channel of 60 mm width, 3 mm height, and 460 mm length, and the dimensions of the graphene-Cu composite test plates are 10 mm × 10 mm. The feed mass fluxes used in the experiments are 240 and 480 kg/m2 s, while the wall heat flux are varied from 0.1 to 20 W/cm2. Based on the experimental analysis, the heat flux slightly increased before the onset of nucleating bubbles. Subsequently, the values drastically increased as the wall superheat continued to increase. Additionally, in experiments with higher graphene concentration, higher heat flux and heat transfer coefficient were demonstrated, and the increase in feed mass flux promoted flow boiling heat transfer.

Flow condensation heat transfer characteristics of R134a in multiport mini-channel by jet impingement cooling

The flow condensation heat transfer of refrigerant R134a inside a multiport mini-channel having a hydraulic diameter of 0.63 mm is experimentally investigated in the present study by jet impingement cooling. Experiments were performed under the operating states of refrigerant vapor quality 0–1, heat flux 10–46 kW/m2, mass flux 115–290 kg/(m2·s), and saturation pressure 0.38–0.51 MPa. The effects of these factors on condensation heat transfer are quantitatively studied. The results indicate that the local heat transfer coefficient is seldom affected by the mass flux and vapor quality of the refrigerant but is closely related with heat flux and saturation pressure. The prediction accuracies of the existing correlations are assessed, and a new formula is proposed. This study on flow condensation heat transfer in a multiport mini-channel is meaningful for the investigation and design of a two-phase mini-channel condenser.

Experimental study of the heat transfer characteristics of supercritical pressure R134a in a horizontal tube

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

An experimental study and empirical correlations to describe the effect of lubricant oil on the nucleate boiling heat transfer performance for R-1234ze and R-134a

In the present work, nucleate boiling heat transfer characteristics of R-134a and R-1234ze refrigerants subject to the influence of lubricant oil on a plate surface were experimentally examined. Eight types of lubricants, including POEA68, POEA120, POEA170, POEB68, POEB120, POEB170, POEC170, and POED 150, were used for investigations; the experiments were carried out at saturated temperatures of 0 °C and 10 °C, respectively. The oil concentrations range from 0% to 10% and heat fluxes vary from 10 kW/m2 to 70 kW/m2. The test results for pure refrigerants are in line with the existing data and correlations. With the presence of lubricant, the heat transfer coefficient may be slightly higher or lower than those of pure refrigerants, depending on the lubricant and operating pressure. However, all the heat transfer coefficients deteriorate appreciably as compared to the pure refrigerants when the oil concentration exceeds 5%. A correlation is developed that is capable of describing the influence of lubricant oil. The proposed correlation not only can predict the influence of the lubricants of this study but also can extend to predict existing data with a good accuracy.

Flow Boiling Heat Transfer of R134a in a Vertical Helically Coiled Tube

ABSTRACTThe boiling heat transfer characteristics of R134a in a helically coiled tube under high heat flux condition were investigated experimentally in present paper. The inner diameter and the coil diameter of the coil were 8 mm and 205 mm, respectively. Experiments were carried out with the heat flux in the range of 10–60 kW/m2, pressure 0.8–1.1 MPa, mass flux 195–400 kg/(m2 s) and vapor qualities 0.1–0.9. The results show that nucleate boiling plays a significant role even at high vapor quality. Inversely, the convective boiling was weakened in the high quality region under high heat flux conditions. The available boiling heat transfer coefficient correlations were compared with present experimental results. A new boiling heat transfer coefficient correlation for R134a in helically coiled tube was proposed based on the boiling mechanisms analysis. The new correlation was also proved to be applicable for high mass flux and low heat flux conditions.

Investigation of boiling heat transfer characteristics of R134a flowing in smooth and microfin tubes

In this study, boiling heat transfer and pressure drop experiments are performed in order to investigate heat transfer enhancement of microfin tube comparing with smooth one. During constant heat flux (10 kW m−2) experiments, effect of mass flux (190–381 kg m−2 s−1), saturation temperature (15–22 °C) and vapor quality (0.21–0.77) on heat transfer coefficient and pressure drop are investigated. In addition, boiling heat transfer coefficient and frictional pressure drop correlations given in the literature are compared with measured ones. Moreover, the experiments revealed that experimental boiling heat transfer coefficient and total pressure drop of R134a flowing microfin tube are 1.9 and 3.0 times higher than smooth one having the same fin root diameter, respectively. Furthermore, 94 number of heat transfer and pressure drop raw experimental data belonging to the microfin tube are given for researchers to validate their theoretical models.

Analyze of laminar flow and boiling heat transfer characteristics of R134a in the horizontal micro-channel under low temperature condition

A numerical investigation has been carried out to examine the characteristics of laminar flow and boiling heat transfer of R134a in horizontal micro-channel with hydraulic diameter of 1mm. Three-dimensional equations considering conjugate heat transfer, viscous heating and temperature-dependent properties are solved for the fluid flow and heat transfer by ANSYS FLUENT. The influence of refrigerant mass velocity, heat flux density and gasification rate on boiling heat transfer was studied. Results show that under the low temperature condition, the total pressure drop increase with the heat flux along the tube, which caused the saturation temperature of refrigerant decrease, and evaporate strengthen, therefore, the total temperature increase at first and then slow down, the heat flux is mainly used in phase-change in this stage. It also can be found that the greater mass flow rate is more advantageous to evaporation.

An experimental study on subcooled flow boiling heat transfer characteristics of R134a in vertical helically coiled tubes

The helically coiled tubes with two-phase flow are widely used as heat transfer equipment in power stations, chemical plants, nuclear reactors, refrigeration systems, and many others. This paper investigates experimentally the subcooled boiling heat transfer characteristics of R134a in vertical helically coiled tubes with heat flux varying from 0.1 to 14.5kWm−2, mass flux from 147.5 to 443.7kgm−2s−1, inlet subcooled temperature from 4.7 to 15.0°C and pressure from 450 to 850kPa. Experimental results indicate that the generated bubbles always slide along the heated surface in the vertical helically coiled tube while the radial component force is negative, which contributes significantly to the heat transfer enhancement and nonuniformity of the wall temperature in subcooled flow boiling. Moreover, the temperature distribution in the vertical helically coiled tube is totally different from that of the horizontal one which is affected by the secondary flow, velocity profile of main flow and bubble behaviors. However, there is no much difference in the parametric effects on the heat transfer coefficient between vertical and horizontal arrangements. The heat transfer coefficients of vertical and horizontal helically coiled tube increase with system pressure, but decrease with subcooling. Based on the experimental data, the correlation of subcooled boiling heat transfer coefficient in helical coil is developed by using dimensional analysis, correlating 96.6% of the data within ±20%.

Heat transfer characteristics of R-134a in a converging-Diverging nozzle

Heat transfer characteristics of cavitating flows in the diverging section of a converging-diverging nozzle, with R-134a as the working fluid were investigated for mass fluxes from 11.3–52.3 kg m−2 s−1 and heat fluxes from 52.6–693 kW m−2. Eleven different nozzle configurations were examined with different divergence angles, throat diameters, inlet profiles, and lengths. Heat flux measurement assumptions were analyzed using numerical conduction simulations with ANSYS Fluent. Experimental results indicated two different heat transfer regimes at high Reynolds number—one high and one low—likely due to wall dryout. A temperature drop with a strong correlation to Reynolds number as well as inlet profile was recorded due to the cavitation-induced phase change. Additionally, data showed two-phase heat transfer coefficients from 3.7 to as high as 285kW m−2 K−1. Chen’s correlation compared well with the recorded heat transfer coefficients for low Reynolds numbers. Variation of heat transfer coefficients with different geometry parameters were given, and potential sonic effects were discussed. With such high heat transfer coefficients, cavitation enhanced heat transfer in converging-diverging nozzles could be of significant use in electronics cooling applications.

Condensation heat transfer characteristics of R134a flowing inside mini circular and flattened tubes

The condensation heat transfer characteristics of R134a flowing in a circular tube and 3 flattened copper tubes are investigated. The flattened tubes were made from round tubes with a 3.51mm inner diameter. The tested tube configurations were as follows: a circular tube with a 3.51mm inner diameter; flattened tube with a 0.72 aspect ratio (FT1); flattened tube with a 3.49 aspect ratio (FT2); and flattened tube with a 7.02 aspect ratio (FT3). The experimental ranges covered a mass flux of 350–900kg/m2s, heat flux of 10–50kW/m2, inlet quality of 0.1–0.9, and saturation pressure of 8–12bars. The flow pattern map was initially investigated by comparing it with existing well-known flow pattern maps. Except for some of the results obtained from FT1, most of the experimental data for all of the tested tubes fell into the categories of semiannular and annular flow patterns. The results revealed that the condensation heat transfer coefficient increased with increasing mass flux, heat flux, and vapor quality. The heat transfer coefficients of the flattened tubes were higher than that of the circular tube, around 5–10%, 10–50%, and 200–400% for FT1, FT2, and FT3, respectively. The existing correlations for predicting the heat transfer coefficient were not successful for the flattened tubes. A new correlation was proposed for practical applications.

Saturated Flow Boiling Heat Transfer Correlation for Small Channels Based on R134a Experimental Data

In the literature, the modeling of saturated flow boiling heat transfer characteristics of R134a flowing through small channels is still questionable since it is based on very limited operational data. This study proposes a new evaporative heat transfer model based on R134a for micro- and macro-tubes. The proposed correlation is developed from 3594 data points, which are obtained from 19 different studies. Ranges of the database cover mass fluxes between 50.0 and 1500.0 kg/m2 s, heat fluxes between 3.0 and 150.0 kW/m2, hydraulic diameter between 0.5 and 13.84 mm, saturation temperatures between −8.8 and \({52.4 ^{\circ} {\rm C}}\), and vapor qualities up to 1.0. Findings of the proposed method are compared with those of the most quoted flow boiling heat transfer correlations developed for micro- and macro-tubes. Results of the comparison indicate that new method, which has a mean absolute deviation of 19.1 % and captures 66.7 and 83.2 % of the experimental data within ±20 and ±30 % error bands correspondingly, outperforms the available flow boiling correlations in the literature in terms of prediction accuracy.

Flow pattern visualization and heat transfer characteristics of R-134a during evaporation inside a smooth tube with different tube inclinations

Flow pattern and heat transfer during evaporation in an 8.9mm diameter smooth tube are presented. The experimental facility makes the tube capable of having different inclination angles, α, with respect to horizon. The experiments were performed for seven different tube inclinations, α, in a range of −90° to +90° and five refrigerant mass velocities in a range of 53 to 170kg/m2s for each tube inclination angle. The experimental results indicate that the tube inclination noticeably influenced the vapor and liquid distribution as well as the evaporation heat transfer coefficient. Seven different flow regimes are observed during evaporation inside a smooth tube with different tube inclinations. In the case of evaporation heat transfer, for all refrigerant mass velocities, the highest heat transfer coefficient is attained at an inclination angle of α=+90°. The effect of tube inclination angle on heat transfer coefficient is more prominent at low vapor quality and mass velocity. A correlation has also been developed to predict the heat transfer coefficient during flow boiling inside a smooth tube with different tube inclinations.