Pressure Drop Of R134a Research Articles

CONDENSATION HEAT TRANSFER IN SMOOTH AND THREE-DIMENSIONAL DIMPLED TUBES OF VARIOUS MATERIALS

The heat transfer and pressure drop of R410A and R32 within a smooth and an enhanced dimpled tube were measured for mass fluxes from 100 to 400 kg m<sup>-2</sup> s<sup>-1</sup>, average vapor qualities between 0.2 and 0.8, and saturation temperatures between 35°C and 45°C. The test section was 2 meters in length, and the outer and inner diameters of the tube were 9.52 and 8.32 mm, respectively. The inner surface of the enhanced tube was dimpled. Three dimpled tubes and three smooth tubes, differing by material (copper, aluminum, and stainless steel) were tested to examine the material effect. The measured condensation heat transfer coefficient (HTC) for the copper smooth tube was 1.10 to 1.16 times higher than that of the aluminum, and likewise, between 1.19 and 1.31 times higher than that of the stainless-steel tube. Similarly, the condensation HTC for the copper dimpled tube was 1.06 to 1.15 times higher than that of the aluminum dimpled tube, and 1.26 to 1.38 times higher than that of stainless-steel dimpled tube. In general, and the condensation HTC for R32 was greater than that for R410A, mainly due to the greater liquid thermal conductivity of R32. Flow patterns were observed for different vapor qualities and used to establish corresponding heat transfer mechanisms. Finally, a new correlation for dimpled tubes was proposed based a modified smooth tube correlation, which predicted the measurements within an error band of 20&#37.

Pressure drop of R134a in mini channels with micro pin fins during flow boiling

Flow boiling in mini channel with micro pin fin arrays is a promising technology for thermal management in various fields such as microelectronics, energy utilization and national defence. Understanding its flow characteristics is the pave stone for further structure optimization of two-phase microchannel heat sinks. However, previous studies paid few attentions to the effect of different structural parameters on its flow performance, especially for the application with long channels for practical evaporators in aircraft environmental control systems. In this paper, an experimental system was set up to investigate the flow boiling pressure drops of refrigerant R134a in a 300 mm long channel with micro pin fin arrays. The flow characteristics of staggered diamond pin fin arrays with different channel widths of 1.0, 1.2 and 1.4 mm and fin angles of 30°, 60°and 90° were comprehensively investigated. The pin fins with a height of 0.5 mm were arranged in the rectangular channel through 3D printing. Two-phase pressure drops in the test section were then measured. The test range of vapor quality, mass flux, heat flux and saturation pressure were 0–1, 200–500 kg/m2s, 10.0–37.5 kW/m2 and 390–700 kPa respectively. Experimental results revealed that the smaller channel width and the greater diamond angle resulted in the greater two-phase pressure drops. The heat flux showed an insignificant influence on the two-phase pressure drops. The existing empirical correlations for pressure drops were evaluated and failed to predict the present experimental data. A new one with the consideration of pin fin structure parameters was then developed to improve the prediction accuracy with the maximum mean absolute deviation of 5.4 %. The proposed correlation can illuminate future study on flow boiling in mini channel with pin fin arrays and be beneficial for the practical design of two-phase microchannel heat sinks.

Simulation Research on Flow Boiling and Heat Transfer of Micro-channel for Electronic Cooling

Micro-channel heat exchanger is a popular device for the electronic cooling. Heat transfer of flow boiling in micro-channel is a research hotspot. For the convenience of the theoretical research on micro-channel, a high efficiency simulation model on flow boiling and heat transfer of micro-channel was established. In the simulation research, the influence rules of operating conditions and thermal properties of refrigerants (working fluids) on the flow boiling were studied; the distribution rules along flow direction of characteristic parameters such as vapor quality, Heat Transfer Coefficient (HTC), pressure drop and bottom wall temperature of micro-channel T w were analyzed; the key factors on the distribution of characteristic parameters were proposed. The characteristic parameters based on the refrigerants R134a and R1234ze(E) in micro-channel were compared and researched. According to the simulation results, HTC and pressure drop of R134a are larger and smaller than those of R1234ze(E) respectively, the uniformities of HTC and T w of R134a are all better than those of R1234ze(E) respectively.

Two-phase frictional pressure drop of R134a and R410A condensing inside multiport rectangular microchannels with different aspect ratio

ABSTRACT This paper presents the experimental result of R134a and R410A (without lubricating oil) pressure drop condensing along three parallel, rectangular microchannels with the aspect ratio varying from 0.5 to 1 and Dh from 0.66 to 1 mm, while maintaining the constant channel depth (1 mm), base area (63 cm2), and channel length (30 cm). All the factors accountable for change in pressure through microchannels were determined for all the configurations separately, i.e. contraction, expansion, frictional, and deceleration. Two distinct saturation temperatures were used in the present experimental study, with vapor quality 0.05 < x < 0.83 and mass flux 200 < G < 600 kg/m2s. Frictional pressure drop estimations were found to be greater than 90% of overall pressure drop. Condensation pressure drop for both the refrigerants increases directly with increasing mass flux and vapor quality, whereas it decreases with the channel aspect ratio and saturation temperature increase. At constant mass flux, R134a has a higher condensation pressure drop than R410A, and the highest value of condensation pressure drop was obtained in MC-1 having an aspect ratio of 0.5, followed by MC-2 and MC-3 having aspect ratios of 0.7 and 1, respectively.

Experimental Study of Horizontal Flow Boiling Heat Transfer Coefficient and Pressure Drop of R134a from Subcooled Liquid Region to Superheated Vapor Region

For the past few years, research in the field of flow boiling heat transfer has gained immense popularity for unravelling the dominant mechanism responsible for controlling heat transfer and identifying a parametric trend for understanding the characteristics of flow boiling heat transfer. This has led to several assumptions and models for predicting heat transfer during flow boiling without any known generalized mechanism. This study therefore seeks to experimentally study the characteristics of heat transfer during flow boiling over a wide range but small increase in vapor quality from a single-phase subcooled region through to a two-phase superheated vapor region. The study was performed with an R134a refrigerant in a single horizontal circular stainless-steel smooth tube that had an internal diameter of 5 mm. In this experimental study, local heat transfer coefficients and frictional pressure drop were measured for low heat fluxes of 4.6–8.5 kW/m2, mass fluxes of 200–300 kg/(m2s), vapor quality from −0.1 to 1.2 and a low constant saturation pressure of 460 kPa. Flow patterns observed during the study were recorded with a high-speed camera at 2000 fps. In covering a wide range of vapor quality, a peak of heat transfer coefficient near a vapor quality of zero and a local minimum observed in the low vapor quality region were observed, and both were sensitive to heat flux and mildly sensitive to mass flux. Generally, at low vapor quality, the heat transfer coefficient deteriorated with vapor quality and this was sensitive to heat flux but insensitive to mass flux and vapor quality, indicating nucleate boiling dominance in low vapor quality regions. In high vapor quality regions, the heat transfer coefficient was sensitive to mass flux and insensitive to heat flux. This indicates the dominance of convective boiling. In the low vapor quality regions, the flow patterns observed were slug and intermittent, while in the high vapor quality region, annular and mist flow patterns were observed. Generally, frictional pressure drop increased with increasing mass flux and vapor quality in the two-phase region.

관경 변화에 따른 R-1234yf의 응축 압력강하에 관한 실험적 연구

In this study, the condensation pressure drop of R-1234yf according to the variation of the tube diameter is experimentally studied and it is intended to be provided as the basic data for the design of the condenser. The experiment measured the pressure drop of R-1234yf by changing the mass flow rate, saturation temperature, and tube diameter. It also compared with the pressure drop of R-134a under the same conditions. As a result, the pressure drop tends to increase as the mass flow rate increases and the tube diameter decreases. In addition, the pressure drop tends to decrease as the saturation temperature increases. As a result of comparing the experimental results and the pressure drop of R-134a, R-134a showed a larger tendency than R-1234yf. As a result of comparing the experimental results with the existing correlation equation, the correlation equation of Miyara et al showed the most similar prediction.

Evaporation Heat Transfer and Pressure Drop of Low-Global Warming Potential Refrigerant HFO-1234yf in 6.95-mm Horizontal Smooth Tube

This study investigated the evaporative heat transfer coefficient and pressure drop characteristics of R-1234yf in a horizontal tube with an inner diameter of 6.95 mm under various experimental conditions. The heat transfer coefficient increased with an increase in quality but showed a sharp decrease in the high-quality area. In addition, the heat transfer coefficient increased as the mass flux, heat flux, and saturation temperature increased. Although R-1234yf and R-134a presented similar heat transfer coefficients, that of R-134a was higher. The pressure drop increased with an increase in the quality and mass flux but decreased with an increase in the saturation temperature. The pressure drop of R-134a was larger than that of R-1234yf. In light of the flow pattern diagram by Taitel and Dukler, most of the experiments were included in the annular flow region, and some regions showed intermittent and stratified corrugated flow regions. Kandlikar’s heat transfer coefficient correlation provided the best prediction for the experimental database, with approximately 84% of the predicted data within ±30%. Moreno Quibén and Thome’s equation for pressure drop predicted approximately 88.71% of the data within ±30%.

An Experimental Investigation on R134a Evaporation inside an Internally Discrete Inclined Grooved Tube

In this experimental study, heat transfer and pressure drop characteristics of R134a evaporating flow are investigated inside a horizontal internally grooved tube with discrete inclined grooves. The test evaporators are made of smooth and grooved copper tubes of 20 mm inner diameter. The length, width, and depth of grooves are 6, 2, and 0.5 mm, respectively, and the angle of the grooves with the tube centerline is 30°. The test conditions include mass flux in the range of 16.5 to 105 kg/m2s, vapor quality of 0.06 to 0.98, and heat flux of 3.5 to 25 kW/m2. The experiment results show that the boiling heat transfer coefficient of R134a in the grooved tube is 1.25 to 1.65 times higher than that of the smooth one. The frictional pressure drop of R134a inside the grooved tube is 10% to 60% higher than the smooth tube. Moreover, by increasing mass flux, the heat transfer coefficient, and frictional pressure drop of R134a refrigerant inside horizontal smooth and grooved tubes increase as well. In conclusion, the energy performance of R134a evaporating flow inside the grooved tube is more efficient than that in the smooth tube regarding energy saving in most experiments.

Numerical study of flow boiling flow patterns and pressure drop of R134a in small tubes under high flight acceleration

A series of numerical simulations of R134a flow boiling in two small tubes with inner diameters of 1 and 2.17 mm under high flight acceleration were conducted based on the VOF model. Six acceleration levels from 1 to 15 g and three directions from 0° to 180° were investigated. At first, the simulated frictional pressure drops under low flight acceleration are compared with previous experimental findings, and the comparison results show a consistent trend. Then, the simulated flow patterns and pressure drops under high flight acceleration with different levels and directions are intercompared, and the influences of high flight acceleration are obtained. The intercomparison results reveal that the flow patterns and pressure drops are influenced by high flight acceleration dramatically. The total and frictional pressure drops decrease with the increasing flight acceleration at θ = 0°, decrease slightly at θ = 90°, and increase at θ = 180°, monotonically, because of the variation of flow patterns. The vapor phases gradually turn into quasi-ellipsoids/spheres, quasi-stratified state, and elongated slugs at θ = 0°, 90°, and 180°, respectively, as the flight acceleration increases.

Investigation of flow boiling heat transfer and pressure drop of R134a in a rectangular channel with wavy fin

The saturated flow boiling heat transfer and pressure drop studies of R134a were experimentally investigated in a rectangular channel with wavy fin. Experiments were performed at mass flux range 30–50 kg m−2 s−1, heat flux range 11–18 kW m−2 and quality 0.26–0.8. The experimental data were obtained in a brazed test section. In preliminary step, single phase experiments were conducted to find out the j and f data of the wavy fin. In second step, two-phase flow boiling experiments were conducted to estimate the heat transfer and frictional coefficient based on experimental data. The trends of heat transfer and pressure drop with respect to mass flux, heat flux and quality were studied. Two-phase local boiling heat transfer coefficient is correlated in terms of Reynolds number factor F, and Martinelli parameter X. Pressure drop is correlated in terms of two-phase frictional multiplier, ϕf and Martinelli parameter, X.

Experimental study on evaporative heat transfer and pressure drop of R-134a in a horizontal dimpled tube

The heat transfer and pressure drop characteristics of R-134a during evaporation in smooth and dimpled tubes are experimentally examined. All test sections used in this study are double-tube heat exchangers, where the hot water in an annulus transfers its heat to R-134a in the inner tube. The experiments are conducted in a mass flux range of 300–500 kg/m2 s, heat flux range of 20–30 kW/m2, and evaporating temperature range of 7–13 °C. Smooth and dimpled tubes with a length of 1500 mm and an inner diameter of 8.1 mm are tested. The results indicate that the tube with protrusions enhance the heat transfer up to 70% along with a pressure drop increase up to 587% over the smooth tube. The effects of relevant parameters on heat transfer and pressure drop are explored. In this experiment, the heat transfer enhancement factor falls in the range of 1.26–1.7, while the pressure drop penalty factor fluctuates between 3.78 and 6.87. In addition, the efficiency index is applied to estimate the performance of dimpled tube. It is found that all efficiency index values are in the range of 0.18–0.42.

Pressure drop of R134a, R32 and R1233zd(E) in diabatic conditions during condensation from superheated vapor

Pressure drop during condensation from superheated vapor is reported in this paper. Three different refrigerants including R134a, R32 and R1233zd(E) are evaluated in two horizontal round copper tubes with 6.1 and 4.0 mm inner diameters. The test conditions cover a range of mass fluxes from 100 kg m−2 s−1 to 400 kg m−2 s−1and heat fluxes from 5 kW m−2 to 15 kW m−2. The condensing pressures are selected to be those that correspond to 30 °C. The measurements are all conducted with diabatic flows which start in the superheated (SH) region to observe how the onset of condensation affects the pressure drop. The visualizations of condensation are compared with the measured pressure drop, suggesting the pressure drop behaviors in different regions during the condensation process are different due to different flow regimes. The onsets of condensation all happens in the superheated region before specific enthalpy drops to that of bulk quality 1. The pressure drop depends on the mass flux, specific enthalpy and tube diameter. The result also shows that several thermal properties of the refrigerants such as the density ratio and viscosity have an effect on the pressure drop, which can be explained by the generation of waves and the differences in vapor and liquid velocity. A peak in the pressure drop curve is consistently present between the onset and end of condensation around bulk quality 1, which could be a result of the competition between increasing liquid-wave strength and decreasing flow velocity as condensation proceeds after the onset of liquid film. Since none of the current existing correlations are sufficient in accounting for the different mechanisms in different stages of the condensation process, a pressure drop model that is specifically developed for condensation of superheated vapor by taking non-equilibrium into consideration is in need.

A complete evaluation method for the experimental data of flow boiling in smooth tubes

A complete solution for boiling phenomena in smooth tubes has been giving as a procedure regarding with the calculation of convective heat transfer coefficient and pressure drop using accurate experimental data validated by flow regime maps and sight glasses on the experimental facility. The experimental study is conducted in order to investigate the effect of operating parameters on flow boiling convective heat transfer coefficient and pressure drop of R134a. The smooth tube having 8.62mm inner diameter and 1100mm length is used in the experiments. The effect of mass flux, saturation temperature and heat flux is researched in the range of 290–381kg/m2s, 15–22°C and 10–15kW/m2, respectively. The experiments revealed that the heat transfer coefficient and pressure drop are significantly affected by mass flux for all tested conditions. Moreover, the experimental results are compared with well-known heat transfer coefficient and frictional pressure drop correlations given in the literature. In addition, 122 number of heat transfer and pressure drop raw experimental data is given for researchers to validate their theoretical models.

Effect of straight micro fins on heat transfer and pressure drop of R410A during evaporation in round tubes

The objective of this paper is to present the influence of straight micro-fins on heat transfer coefficient and pressure drop. R410A flow boiling experiments were conducted in both smooth tube and the axial micro-finned tube. Data are taken at 10 °C saturation temperature for the vapor quality from 0.1 to 0.9, mass flux from 100 to 450 kg/s-m2, and heat flux from 10 to 20 kW/m2. The results in the smooth tube were compared with several correlations and used as a baseline. It was observed that straight micro-fin structure played an important role in the evaporative heat transfer. The heat transfer coefficient was significantly enhanced in the axial micro-finned tube, whose enhancement factor ranged from 1.03 to 1.48 with average of 1.34. As the mass flux and vapor quality was low, the straight micro-fins had stronger influence of the heat transfer coefficient since the liquid-phase refrigerant was easily trapped in the grooves of the axial micro-finned tube. The pressure drop penalty factor of the axial micro-finned tube ranged from 0.66 to 1.6, and the average was around 1.23.

Condensation heat transfer and pressure drop of R-410A in flat aluminum multi-port tubes

Brazed heat exchangers with aluminum flat multi-port tubes are being used as condensers of residential air-conditioners. In this study, R-410A condensation tests were conducted in four multi-port tubes having a range of hydraulic diameter (0.78 ≤ Dh ≤ 0.95 mm). The test range covered the mass flux from 100 to 400 kg/m2 s and the heat flux at 3 kW/m2, which are typical operating conditions of residential air conditioners. Results showed that both the heat transfer coefficient and the pressure drop increased as the hydraulic diameter decreased. The effect of hydraulic diameter on condensation heat transfer was much larger than the predictions of existing correlations for the range of investigation. Comparison of the data with the correlations showed that some macro-channel tube correlations and mini-channel tube correlations reasonably predicted the heat transfer coefficient. However, macro-channel correlations highly overpredicted the pressure drop data.

Measurement of heat transfer coefficient and pressure drop during evaporation of R134a in new type facility with one pass flow through microchannel tube

A new facility has been built for experimental study on microchannel and introduced in this paper. Six heat transfer coefficient and six pressure drop measurements can be conducted simultaneously in one pass from vapor quality of 0 to 1 on the facility. Two water-jacket-design heat exchangers are placed top and bottom on each test section for heating the refrigerant. In each test section, the top and bottom sides are heated and controlled separately to ensure uniform conditions. The environmental heat loss and wall temperature measurements are corrected to increases confidence of results. Port geometry is measured with an optical microscope. Heat transfer coefficient and pressure drop of R134a are measured in a 24-port microchannel tube. The single-phase pressure drop follows the rule of round tube and fRe is 64. The single-phase Nusselt number is more sensitive to Reynolds number, compared to large tubes. As vapor quality increases, two-phase pressure drop increases until quality of 0.9, and then stops increasing or even drops; two-phase heat transfer coefficient increases until quality of 0.5 to 0.6 and then drops. Higher mass flux or lower saturation temperature will increase the pressure drop. Heat flux and mass flux have strong impact on heat transfer coefficient, and the effect of saturation temperature is not clear.

An Experimental Investigation of Condensation Heat Transfer Coefficients and Pressure Drops of Refrigerants Inside Multiport Mini-channel Tubes

The condensation heat transfer coefficients and pressure drops of R410A and R22 flowing inside a horizontal aluminum multiport mini-channel tube having 18 channels are investigated. Experimental data are presented for the range of vapor quality from 0.1 to 0.9, mass flux from 50 to 500[Formula: see text]kg/m2s, heat flux from 3 to 15[Formula: see text]kW/m2 and the saturation temperature at 48[Formula: see text]C. The pressure drop across the test section was directly measured by a differential pressure transducer. At a small scale, the noncircular cross-sections can enhance the effect of the surface tension. The average heat transfer coefficient increased with the increase of vapor quality, mass flux and heat flux. Under the same test conditions, the heat transfer coefficients of R22 are higher than those for R410A, the pressure drops for R410A are 7–19% lower than those of R22. The lower pressure drop of R410A has an important advantage as an alternative working fluid for R22 in air-conditioning and heat pump systems.

Investigation on condensation heat transfer and pressure drop of R410A during upward flow in vertical smooth and micro-fin tube

Heat transfer and pressure drop performance of refrigerant R410A condensation during vapor flowing upward in a smooth tube with inner diameter of 8.32mm and a micro-fin tube with fin root diameter of 8.76mm are experimentally investigated. The test section is a 0.5m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus in the same direction. Experiments are conducted at saturation temperatures 40°C, 45°C and 48°C with mass fluxes in the range of 80–345kg/m2s. Four different cooling water flow conditions in the test section are set up to compare the heat transfer enhancement factors and pressure drop penalty factors of the smooth tube and micro-fin tube. The total pressure drops across the test section are directly measured by a differential pressure transducer. The experimental data are compared with some well-known heat transfer correlations of micro-fin tube. The correlation suggested by Chamra and Mago shows the best agreement with the present data and exhibits an average deviation of 31.08 and a mean deviation of 35.74. Based on the experimental data, two revised correlations for condensation heat transfer and the frictional pressure drop in micro-fin tube are proposed, within ±15% and ±25% deviation band respectively.

冷媒R410Aのアルミニウム製溝付管内の凝縮・蒸発熱伝達および圧力損失

冷媒R410Aのアルミニウム製溝付管内の凝縮・蒸発熱伝達および圧力損失

Experimental study of boiling heat transfer in smooth/micro-fin tubes of four refrigerants

An experimental investigation of boiling characteristics in a horizontal smooth and micro-fin tube with 9.52mm outside diameter and 1m length was conducted. The refrigerants tested were R22, R134a, R407C and R410A while vapour quality ranges from 0.1 to 0.9, mass flux 50, 250, 450kgm−2s−1 and heat flux of 5, 12.5, 20kWm−2. The saturation temperature is 5°C. For the smooth tube, the average heat transfer coefficients of R134a, R407C and R410A are 110.9%, 78.0% and 125.2% of those of R22 in test conditions respectively. For the micro-fin tube, the average heat transfer coefficients of R22, R134a, R407C and R410A are 1.86, 1.80, 1.69 and 1.78 times higher than those of the smooth tube. The pressure drop of R22, R407C and R410A for the smooth tube is similar to each other while the pressure drop of R134a is 1.7 times higher. The average pressure drop of R22, R134a, R407C and R410A for the micro-fin tube is 1.42, 1.30, 1.45 and 1.40 times higher when compared with that for the smooth one. Considering the effect of heat transfer enhancement and pressure drop augment, the efficiency index η1 which values the thermo-hydraulic performance at identical flow rate of R22, R134a, R407C and R410A in the micro-fin tube used is 1.31, 1.38, 1.17 and 1.27 respectively compared with the smooth tube.