Experimental two-phase heat transfer coefficient and frictional pressure drop inside mini-channels during condensation with R1234yf and R134a (original) (raw)
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R1234YF Heat Transfer Coefficient During Condensation in a Mini-Channel Multiport Tube
Proceedings of the 15th International Heat Transfer Conference, 2014
The use of micro-and mini-channels in heat exchanger has increased in recent decades. They contribute to increasing efficiency and to reducing refrigerant charge and compactness of heat exchangers. The aim of this study is to experimentally determine heat transfer coefficient in mini-channels two-phase flow processes with the low GWP refrigerant R32 and compare it with the values provided by some of the correlations encountered in the existing literature. In the existing literature there are a few publications studying the refrigerant R32. R32 has medium flammability, classified as A2 by ASHRAE. European air conditioning manufactures point to use R32 instead of R410A. R32 has lower global warming potential (GWP = 675) than R410a (GWP = 2088). Environmental improvements must be also considered. R32 is a single component refrigerant so recycling is easier than R410A process; also R32 is safer than R410A by NFPA 704 classification; with R410A a breathing apparatus is required in case of accident. On the other hand, R410A flammability is lower than R32 because of the addition of R125. An installation for the study of condensation processes has been constructed at the "Technical University of Cartagena", Spain. The more relevant results of heat transfer coefficient will be presented in this paper. The analysed data have been measured for R32 flowing through aluminium square multiport tubes with a hydraulic diameter of 1.16 mm and compared with R410A. The influence of saturation temperature (or pressure), flow velocity, and vapour quality in heat transfer coefficient and frictional pressure gradient have been studied. The values considered for these variables are: saturation pressure corresponding to 30, 35, 40, 45, and 50ºC; flow velocities from 100 to 800 kg/(s•m 2); vapour quality from 0.05 to 0.9.
Experimental Thermal and Fluid Science, 2011
In the present experimental study, condensation of steam inside helically coiled tubes was investigated. Three helical coils of coil diameter 110, 135 and 160 mm were tested. The effect of mass flux, coil curvature ratio and temperature difference between steam and tube inner wall on the condensation heat transfer coefficient were analysed. The condensation heat transfer coefficient increases with the increase in the value of mass flux. Temperature difference has substantial effect on the heat transfer coefficient. For the first time, the effect of coil curvature ratio on condensation heat transfer is studied and discussed in the paper. It is found that helical coil having greater coil curvature ratio has high values of condensation heat transfer coefficient in comparison to a low curvature ratio. Experimental results are compared with the work of earlier investigators. A new empirical correlation is developed to predict the condensation heat transfer coefficient.
This thesis presents a detailed study of parametric characterization of two-phase condensing flow of two selected refrigerants R134a and R-245fa in a single water-cooled micro-channel of 0.4 mm X 2.8 mm cross-section (0.7 mm hydraulic diameter and 7:1 aspect ratio) and 190 mm in length. To avoid flow mal-distribution associated with typical micro-channel tube banks, a single micro-channel was fabricated utilizing an innovative approach and used for the present study experiments. The study investigated the effects of variations in saturation temperature ranging from 30 o C to 70 o C, mass flux from 50 to 500 kg/m 2 s, and inlet super heat from 0 o C to 15 o C on the average heat transfer and overall pressure drop coefficient of the micro-channel condenser. In all cases the inlet vapor quality was kept at 100% quality (saturated vapor) and the outlet condition was always kept at 0% quality (saturated liquid). Accuracy of the fabricated channel geometry with careful design
International Journal of Heat and Mass Transfer, 2012
This paper presents the results of an investigation of the influence of hydrodynamic instabilities on heat transfer intensity during the condensation of R134a and R404A refrigerants in pipe mini-channels. The heat transfer coefficient h is a measure of the effectiveness of the condensation process. It is particularly important to determine the value of the coefficient in the two-phase condensation area in a compact condenser. In other condenser areas (i.e., precooling of superheated vapor and subcooling of condensate), the heat efficiency is substantially smaller. Hydrodynamic instabilities of a periodic nature have an influence on size changes in these areas. A decrease in the heat transfer coefficient h in the two-phase area results in decreased intensity of the heat removal process in the whole condenser. The experimental investigations were based on the condensation of R134a and R404A refrigerants in horizontal pipe mini-channels with internal diameters of d = 0.64; 0.90; 1.40; 1.44; 1.92; 2.30 and 3.30 mm. Disturbances of the condensation process were induced with a periodic stop and a repetition of the flow of the refrigerant. In the range of frequencies, f = 0.25-5 Hz, of the periodically generated disturbances, an unfavorable influence on the intensity of the heat transfer during the condensation process in pipe mini-channels was identified. The reduction in the intensity of the heat transfer during the condensation process, which was induced with hydrodynamic instabilities, was presented in the form of the dependence of the heat transfer coefficient h on the vapor quality x and the frequencies f of the disturbances. The influence of the refrigerant, the diameter of the mini-channels and the frequency f on the damping phenomenon of the periodical disturbances in the pipe mini-channels was identified.
Two Phase Heat Transfer Coefficient for Minichannel Condensers
SSRN Electronic Journal, 2017
Vapour compression refrigeration (VCR) system was designed, developed and fabricated for testing alternative refrigerants such as R152a, R600a, R290 and mixture of R290/R600a (50/50 by wt %) over presently used R134a with aluminium minichannel condenser. In the test set up, heater bank was provided for controlling condensation temperature and evaporator temperature along with sub-cooling and superheating temperature by using PID controllers. All the refrigerants were tested for condensation temperature ranging from 40 to 55 °C while evaporation temperature ranges from-15 to 15 °C. Refrigerant charge was reduced drastically with the minichannel condenser over the conventional condenser. Two phase condensation heat transfer coefficient correlation was developed from the experimental data points to design minichannel and conventional condenser and found in good agreement with the existing well known correlations.
Update on Condensation Heat Transfer and Pressure Drop inside Minichannels
The present paper reviews published experimental work focusing on condensation flow regimes, heat transfer, and pressure drop in minichannels. New experimental data are available with high (R410A), medium (R134a), and low (R236ea) pressure refrigerants in minichannels of different cross-section geometries and with hydraulic diameters ranging from 0.4 to 3 mm. Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi-empirical and theoretical models developed for conventional channels and models specifically created for minichannels. Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film.
C-2006 An Experimental Study of Condensation Heat Transfer and Pressure Drop.pdf
Only recently, experimental data is available in open literature in condensation of various refrigerants in small hydraulic diameter microchannels. The phenomenon of twophase flow and heat transfer mechanism in small diameter microchannels (< 1 mm) may be different than that in conventional tube sizes due to increasing dominance of several influencing parameters like surface tension, viscosity etc. This paper presents an on-going experimental study of condensation heat transfer and pressure drop of refrigerant R134a is a single high aspect ratio rectangular microchannel of hydraulic diameter 0.7 mm and aspect ratio 7:1. This data will help explore the condensation phenomenon in microchannels that is necessary in the design and development of small-scale heat exchangers and other compact cooling systems. The inlet vapor qualities between 20% and 80% and mass fluxes of 1 30 and 200 kg/m 2 s have been studied at present. The microchannel outlet conditions are maintained at close to thermodynamic saturated l iquid state through a careful experimental procedure. A unique process for fabrication of the microchannel involving milling and electroplating steps has been adopted to maintain the channel geometry close to design values. Measurement instruments are well-calibrated to ensure low system energy balance error, uncertainty and good repeatability of test data. The trends of data recorded are comparable to that found in recent literature on similar dimension tubes.
C-2007 STUDIES ON CONDENSATION OF REFRIGERANTS IN A HIGH ASPECT RATIO MINICHANNEL.pdf
Three different refrigerants, R134a, R245fa and HFE7100 were analyzed as working fluids for two-phase cooling of high heat flux electronics in a 0.7 mm hydraulic diameter 190 mm long high aspect ratio minichannel and in a newly developed micro-groove surface condenser. The latter comprised of a micro-groove surface with rectangular grooves of 84 µm in hydraulic diameter with an aspect ratio of 10.6 and headers that directed the refrigerant flow into the grooves. It was concluded that in the minichannel R245fa provides higher heat transfer coefficients compared to R134a with a significantly higher pressure drop. The saturation temperature drop in the same channel created a significant temperature drop for HFE7100 that make the application of such minichannels for cross-flow condensers with this fluid unpractical. The microgroove surface condenser provided significantly higher heat transfer coefficients compared to the minichannel condenser. The pressure drop in the micro-groove surface condenser was extremely low and imposed just 1C temperature drop on HFE7100 at it highest heat flux. The mass flux of refrigerant in the micro-groove surface condenser is significantly lower compared to conventional mini and microchannel condensers. In its current configuration, the microgroove surface condenser benefits from the possibility of an increase in mass flux resulting in a significant increase in heat transfer coefficient and just a moderate increase in pressure drop.