Condensation heat transfer characteristics of R-22, R-134a and R-410A in small diameter tubes

The condensation heat transfer of pure refrigerants, R-22, R-134a and a binary refrigerant R-410A flowing in small diameter tubes was investigated experimentally. The condenser is a countflow heat exchanger which refrigerant flows in the inner tube and cooling water flows in the annulus. The heat exchanger is smooth, horizontal copper tube of 1.77, 3.36 and 5.35 mm inner diameter, respectively. The length of heat exchanger is 1220, 2660 and 3620 mm, respectively. The experiments were conducted at mass flux of 200–400 kg/m² s and saturation temperature of 40°C. The main results were summarized as follows: in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski and Wu and Little correlation. The new single-phase correlation based on the experimental data was proposed in this study. In case of two-phase flow, the condensation heat transfer coefficient of R-410A for three tubes was slightly higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. The condensation heat transfer coefficient for R-22, R-134a and R-410A increased with increasing mass flux and decreasing tube diameter. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensation heat transfer. Therefore, the new condensation heat transfer correlation based on the experimental data was proposed in the present study.     

Condensation heat transfer and pressure drop characteristics of R-290, R-600a, R-134a and R-22 in horizontal tubes

This paper presents the experimental results of condensation heat transfer coefficients of hydrocarbon (HC) refrigerants R-290 and R-600a, hydrochlorofluorocarbon (HCFC) refrigerant R-22, and hydrofluorocarbon (HFC) refrigerant R-134a in a horizontal double-pipe heat exchanger having pipe inner diameters of 10.07, 7.73, 6.54, and 5.80 mm. The condensation process experiments were conducted at mass flux of 35.5–210.4 kg/m² s and condensation temperature of 40°C. The main results were summarized as follows: The average condensation heat transfer coefficients of R-290 and R-600a were higher than those of R-22 and R-134a. The pressure drops of the four refrigerants were in the order of R-600a > R-290 > R-134a > R-22. The pressure drops of R-600a, R-290, R-134a, and R-22 were approximately 6–15, 9.8–12.5, 4.3–6.7, and 2.1–4.6% higher, respectively, in the 10.7 mm diameter tubes compared to the 5.80 mm diameter tubes. Comparing the condensation heat transfer coefficients of our experimental results with those of other correlations, our experimental data in all the test tubes coincided best with that of Haraguchi et al.     

Condensation pressure drop of R22, R134a and R410A in a single circular microtube

The condensation pressure drop characteristics for pure refrigerants R22, R134a, and a binary refrigerant mixture R410A without lubricating oil in a single circular microtube were investigated experimentally. The test section consists of 1,220?mm length with horizontal copper tube of 3.38?mm outer diameter and 1.77?mm inner diameter. The experiments were conducted at refrigerant mass flux of 450–1,050?kg/m²s, and saturation temperature of 40°C. The main experimental results showed that the condensation pressure drop of R134a is higher than that of R22 and R410A for the same mass flux. The experimental data were compared against 14 two-phase pressure drop correlations. A new pressure drop model that is based on a superposition model for refrigerants condensing in the single circular tube is presented.     

Two-phase flow of R-134a refrigerant during flow boiling through a horizontal circular mini-channel

This research focuses on heat transfer to R-134a during flow boiling in a 1.75 mm internal diameter tube. Flow visualisation and heat transfer experiments are conducted to obtain heat transfer coefficients for different flow patterns. The measured data in each flow regime are compared with predictions from a three-zone flow boiling model. The calculations are in fair agreement with the experimental results which correspond in particular to slug flow, throat-annular flow and churn flow regimes under conditions of low heat flux.     

Evaporation flow pattern and heat transfer of R-22 and R-134a in small diameter tubes

The flow patterns and heat transfer coefficients of R-22 and R-134a during evaporation in small diameter tubes were investigated experimentally. The evaporation flow patterns of R-22 and R-134a were observed in Pyrex sight glass tubes with 2 and 8 mm diameter tube, and heat transfer coefficients were measured in smooth and horizontal copper tubes with 1.77, 3.36 and 5.35 mm diameter tube, respectively. In the flow patterns during evaporation process, the annular flows in 2 mm glass tube occurred at a relatively lower vapor quality compared to 8 mm glass tube. The flow patterns in 2 mm glass tube did not agree with the Mandhane’s flow pattern maps. The evaporation heat transfer coefficients in the small diameter tubes (d _i  < 6 mm) were observed to be strongly affected by tube diameters, and to differ from those in the large diameter tubes. The heat transfer coefficients of 1.77 mm tube were higher than those of 3.36 mm and 5.35 mm tube. Most of the existing correlations failed to predict the evaporation heat transfer coefficient in small diameter tubes. Therefore, based on the experimental data, the new correlation is proposed to predict the evaporation heat transfer coefficients of R-22 and R-134a in small diameter tubes.     

Heat transfer and pressure drop of R-134a condensation in a coiled,double tube

An experimental study was carried out to investigate condensation heat transfer and pressure drop characteristics of R-134a in a coiled double tube oriented with its helix axis in the vertical direction. Measurements were obtained at inlet pressure of 815 kPa for refrigerant mass flux ranging from 95 to 710 kg/m²s and cooling water Reynolds number varying from 1000 to 14000. Presented results illustrate the effects of refrigerant mass flux and average condensation temperature difference on the condensation heat transfer coefficient and pressure drop. Comparison with relevant data from other sources indicates a reasonable agreement. An empirical correlation was obtained for predicting condensation heat transfer coefficient. The present study may be considered of a practical and theoretical interest for the design of the helical double-tube condensers using R-134a as the working fluid. M. El-Sayed Mosaad is on leave from Mechanical Engineering Department, Mansoura University, Egypt.     

Experimental study on condensation heat transfer in vertical minichannels for new refrigerant R1234ze(E) versus R134a and R236fa

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

Subcooled flow boiling of R-134a in vertical channels of small diameter

Subcooled flow boiling heat transfer for refrigerant R-134a in vertical cylindrical tubes with 0.83, 1.22 and 1.70 mm internal diameter was experimentally investigated. The effects of the heat flux, q″ = 1–26 kW/m², mass flux, G = 300–700 kg/m² s, inlet subcooling, ΔT_sub,i = 5–15 °C, system pressure, P = 7.70–10.17 bar, and channel diameter, D, on the subcooled boiling heat transfer were explored in detail. The results are presented in the form of boiling curves and heat transfer coefficients. The boiling curves evidenced the existence of hysteresis when increasing the heat flux until the onset of nucleate boiling, ONB. The wall superheat at ONB was found to be essentially higher than that predicted with correlations for larger tubes. An increase of the mass flux leads, for early subcooled boiling, to an increase in the heat transfer coefficient. However, for fully developed subcooled boiling, increases of the mass flux only result in a slight improvement of the heat transfer. Higher inlet subcooling, higher system pressure and smaller channel diameter lead to better boiling heat transfer. Experimental heat transfer coefficients are compared to predictions from classical correlations available in the literature. None of them predicts the experimental data for all tested conditions.     

Flow boiling heat transfer and pressure drop of R-134a in a mini tube: an experimental investigation

This paper presents the results of an experimental study carried out with R-134a during flow boiling in a horizontal tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m², (ii) the refrigerant mass velocities set to the discrete values in the range of 240-930 kg/(m² s) and (iii) saturation temperature of 12 and 22 °C. The study analyzed the heat transfer, through the local heat transfer coefficient along of flow, and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux in the heat transfer coefficient and mass velocity in the pressure drop, besides the effects of saturation temperature. In the low quality region, it was possible to observe a significant influence of heat flux on the heat transfer coefficient. In the high vapor quality region, for high mass velocities, this influence tended to vanish, and the coefficient decreased. The influence of mass velocity in the heat transfer coefficient was detected in most tests for a threshold value of vapor quality, which was higher as the heat flux increased. For higher heat flux the heat transfer coefficient was nearly independent of mass velocity. The frictional pressure drop increased with the increase in vapor quality and mass velocity. Predictive models for heat transfer coefficient in mini channels were evaluated and the calculated coefficient agreed well with measured data within a range 35% for saturation temperature of 22 °C. These results extend the ranges of heat fluxes and mass velocities beyond values available in literature, and add a substantial contribution to the comprehension of boiling heat transfer phenomena inside mini channels.     

Evaporation heat transfer of R-134a inside a microfin tube with different tube inclinations

An experimental investigation has been carried out to study the heat transfer characteristics during evaporation of R-134a inside a single helical microfin tube. The microfin tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of −90° to +90° and four mass velocities of 53, 80, 107 and 136 kg/m² s for each tube inclination angle during evaporation of R-134a. The results demonstrate that the tube inclination angle, α, affects the boiling heat-transfer coefficient in a significant manner. For all refrigerant mass velocities, the best performing tube is that having inclination angle of α = +90°. The effect of tube inclination angle, α, on heat-transfer coefficient, h, is more prominent at low vapor quality and mass velocity. An empirical correlation has also been developed to predict the heat-transfer coefficient during flow boiling inside a microfin tube with different tube inclinations.     

Flow boiling heat transfer of R134a in the multiport minichannel heat exchangers

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 1.1 mm for 14 numbers of channels and 1.2 mm for eight numbers of channels. The outer surface areas of two inner test sections are 5979 mm² and 6171 m², while the inner surface areas are 13,545 mm² and 8856 mm² 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 25.4 mm. The experiments were performed at the heat fluxes between 15 and 65 kW/m², mass flux of refrigerant between 300 and 800 kg/m² s and saturation pressure ranging from 4 to 6 bar. 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.     

Two-phase heat transfer characteristics for R-22/R-407C in a 6.5-mm smooth tube

Two-phase friction and heat transfer characteristics for R-22/R-407C inside a 6.5-mm smooth tube are reported in this study. The heat transfer results for G=100 and 400 kg/m² s were reported in the present study, and the adiabatic frictional pressure drop was recorded in the range of 100 to 700 kg/m² s. It is found that the development of flow pattern for R-407C falls behind R-22. This may explain the lower pressure drops for R-407C. The major heat transfer mechanism at low mass flux is nucleate boiling, and virtually becomes the convective evaporation as mass flux increase to G=400 krg/m² s, Meanwhile, the reduction of heat transfer coefficients for R-407C mixtures are especially profound at low mass flux, and the reduction of heat transfer coefficient decreases with the increase of mass flux.     

Flow regimes and two-phase pressure gradient in horizontal straight tubes: Experimental results for HFO-1234yf, R-134a and R-410A

Two-phase flow regime visualizations of HFO-1234yf and R-134a in a 6.70 mm inner diameter glass straight tube have been simultaneous investigated by top and side views with a high speed high resolution camera. No major difference was observed between both refrigerants. HFO-1234yf flow regimes were satisfactorily predicted by the Wojtan et al. [1] flow pattern map. In addition, 819 pressure drop data points measured during two-phase flow of refrigerants HFO-1234yf, R-134a and R-410A in horizontal straight tubes are presented. The tube diameter (D) varies from 7.90 to 10.85 mm. The mass velocity ranges from 187 to 1702 kg m⁻² s⁻¹ and the saturation temperatures from 4.8 °C to 20.7 °C. The results are compared against 10 well-known two-phase frictional pressure drop prediction methods. For the entire database, the best accuracy is given by the method of Müller-Steinhagen and Heck [2] with around 90% of the data predicted within a ±30% error band. An analysis was carried out on the maximum pressure gradient and on the corresponding vapor quality. A statistical analysis for each flow regime was also carried out.     

Experimental study of inverted annular film boiling in a vertical tube cooled by R-134a

An experimental investigation of inverted annular film boiling heat transfer has been performed for vertical up-flow in a round tube. The experiments used R-134a coolant and covered a pressure range of 640–2390 kPa (water equivalent range: 4000–14,000 kPa) and a mass flux range of 500–4000 kg m⁻² s⁻¹ (water equivalent range: 700–5700 kg m⁻² s⁻¹). The inlet qualities of the tests ranged from −0.75 to −0.03. The hot-patch technique was used to obtain the subcooled film boiling measurements. It was found that the heat transfer vs. quality curve can be divided into four different regions, each characterized by a different mechanisms and trends. These regions are dependent on pressure, mass flux and local quality. A detailed examination of the parametric trends of the heat transfer coefficient with respect to mass flux, inlet quality, heat flux and pressure was performed; reasonably good agreement between observed trends and those reported in the literature were noted.     

Augmentation of heat transfer by twisted tape inserts during condensation of R-134a inside a horizontal tube

An experimental investigation has been carried out to find the heat transfer coefficient during condensation of R-134a vapor inside a horizontal tube. Experiments were conducted for the condensation of R-134a inside a plain tube and tubes with different twisted tape inserts. Twisted tapes with different twisted ratios of 6, 9, 12 and 15 were inserted in the refrigerant side, one by one, in the full length of test-condenser. For each inserted tube and the plain tube, test runs were carried out for the mass velocities of 92, 110, 128 and 147 kg/s-m². An empirical correlation has also been developed to predict the enhanced heat transfer coefficient.     

Evaporation heat transfer and friction characteristics of R-134a flowing downward in a vertical corrugated tube

Differently from most previous studies, the heat transfer and friction characteristics of the pure refrigerant HFC-134a during evaporation inside a vertical corrugated tube are experimentally investigated. The double tube test sections are 0.5 m long with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tubes are one smooth tube and two corrugated tubes, which are constructed from smooth copper tube of 8.7 mm inner diameter. The test runs are performed at evaporating temperatures of 10, 15, and 20 °C, heat fluxes of 20, 25, and 30 kW/m², and mass fluxes of 200, 300, and 400 kg/m² s. The 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 measured directly by a differential pressure transducer. The effects of heat flux, mass flux, and evaporation temperature on the heat transfer coefficient and two-phase friction factor are also discussed. It is found that the percentage increases of the heat transfer coefficient and the two-phase friction factor of the corrugated tubes compared with those of the smooth tube are approximately 0-10% and 70-140%, respectively.     

Effect of EHD on heat transfer enhancement during two-phase condensation of R-134a at high mass flux in a horizontal smooth tube

In this study, effect of electrohydrodynamic (EHD) on the condensation heat transfer enhancement and pressure drop of pure R-134a are experimentally investigated. The test section is a 2.5 m long counterflow double tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth horizontal copper tubing of 9.52 mm outer diameter. The electrode is made from stainless steel wire of 1.47 mm diameter. The test runs are performed at average saturated temperatures ranging between 40 and 60°C, mass flux ranging between 200 and 600 kg/m² s, heat flux ranging between 10 and 20 kW/m² and applied voltage at 2.5 kV. For the presence of the electrode, the experimental results indicate that the maximum heat transfer enhancement ratio is around 30% while the maximum increase in pressure drop is about 25%.     

Effects of heat flux,mass flux,vapor quality,and saturation temperature on flow boiling heat transfer in microchannels

Flow boiling heat transfer with the refrigerants R-134a and R-245fa in copper microchannel cold plate evaporators is investigated. Arrays of microchannels of hydraulic diameter 1.09 and 0.54 mm are considered. The aspect ratio of the rectangular cross section of the channels in both test sections is 2.5. The heat transfer coefficient is measured as a function of local thermodynamic vapor quality in the range −0.2 to 0.9, at saturation temperatures ranging from 8 to 30 °C, mass flux from 20 to 350 kg m⁻² s⁻¹, and heat flux from 0 to 22 W cm⁻². The heat transfer coefficient is found to vary significantly with heat flux and vapor quality, but only slightly with saturation pressure and mass flux for the range of values investigated. It was found that nucleate boiling dominates the heat transfer. In addition to discussing measurement results, several flow boiling heat transfer correlations are also assessed for applicability to the present experiments.     

Condensation pressure drop of HFC-134a and R-404A in a smooth and micro-fin U-tube

The frictional pressure drop during condensation of HFC-134a and R-404A in a smooth (8.56 mm ID) and micro-fin U-tubes (8.96 mm ID) are experimentally investigated. Different from previous studies, the present experiments are performed for various condensing temperatures. The test runs are done at average saturated condensing temperatures ranging from 35 °C to 60 °C. The mass fluxes are between 90 and 800 kg/m²s. The experimental results indicate that the average frictional pressure drop increases with mass flux but decreases with increasing condensing temperature for both smooth and micro-fin-tubes. The average frictional pressure drops of HFC-134a and R-404A for the micro-fin-tubes were 1-1.7 and 1-2.1 times larger than that in smooth tube respectively. New correlations based on the data gathered during the experimentation for predicting frictional pressure drop are proposed for wide range of operating conditions.     

Flow condensation heat transfer characteristics of hydrocarbon refrigerants and dimethyl ether inside a horizontal plain tube

Flow condensation heat transfer coefficients (HTCs) and pressure drop of R22, propylene, propane, DME and isobutane are measured on a horizontal plain tube. The main test section in the experimental flow loop is made of a plain copper tube of 8.8 mm inner diameter and 530 mm length. The refrigerant is cooled by passing cold water through the annulus surrounding the test section. Tests are performed at a fixed refrigerant saturation temperature of 40 ± 0.2 °C with mass fluxes of 100, 200, and 300 kg/m² s and heat flux of 7.3–7.7 kW/m². The heat transfer and pressure drop data are obtained in the vapor quality range of 10–90%. Test results show that for a given mass flux the flow condensation HTCs of propylene, propane, DME and isobutane are higher than those of R22 by up to 46.8%, 53.3%, 93.5% and 61.6%, respectively. Also well-known correlations developed based upon conventional fluorocarbon refrigerants predict the present data within a mean deviation of 33%. Finally, the pressure drop increases as the mass flux and quality increase and isobutane shows the highest pressure drop due to its lowest vapor pressure among the fluids tested.