Refrigerant 134a liquid flow through micro-scale short tube orifices with/without phase change (original) (raw)
Related papers
A semi-empirical model of two-phase flow of refrigerant-134a through short tube orifices
Experimental Thermal and Fluid Science, 1994
• Measurements were conducted on Refrigerant-134a flowing through short tube orifices with length-to-diameter (L/D) ratios ranging from 5 to 20. Both two-phase and subcooled liquid flow conditions entering the short tube were examined for upstream pressures ranging from 896 to 1448 kPa and for qualities as high as 10% and subcoolings as high as 13.9°C. Data were analyzed as a function of the main operating variables and tube geometry. Semi-empirical models for both single-and two-phase flow at the inlet of the short tubes were developed to predict the mass flow of Refrigerant-134a through short tube orifices. Choked flow conditions for Refrigerant-134a were typically established when downstream pressures were reduced below the saturation pressure corresponding to the inlet temperature. The flow rate strongly depended on the upstream pressure and upstream subcooling/quality. The mass flow also depended on cross-sectional area and short tube length. The mass flow model utilized a modified orifice cquation that formulated the mass flow as a function of normalized operating variables and short tube geometry. For a two-phase flow entering the short tube, the modified orifice equation was corrected using a theoretically derived expression that related the liquid portion of the mass flow under two-phase conditions to a flow that would occur if the flow were a single-phase liquid. It was found that for sharpcdged short tubes with single-and two-phase flow, approximately 95% of the measured data and model's prediction were within _+ 5°/~ of each other.
Convective boiling pressure drop of refrigerant R-134a in horizontal smooth and microfin tubes
International Journal of Refrigeration, 2004
Present study deals with the pressure drop of refrigerant R-134a under convective boiling conditions in horizontal smooth and microfinned ('grooved') copper tubes. Experiments have been carried out in an experimental set up developed for change of phase studies with a test section made out of 7.0, 7.93, and 9.52 mm external diameter, 1.5 m long copper tubes, electrically heated by tape resistors wrapped on the external surface. Mass velocities and refrigerant qualities varied in the following ranges: 70 -1100 kg s 21 m 22 and 5 -95%. The annular flow pattern has been observed to occur over most of the operational conditions. For smooth tubes, the Jung and Radermacher correlation for the liquid two phase flow multiplier fits with reasonable precision the experimental data. As for grooved tubes, a correlation of the two phase flow multiplier in terms of the Martinelli's parameter has been developed which fits the data with an average absolute deviation of the order of 6.3%. The proposed correlation fits with good precision data obtained elsewhere for grooved tubes of different diameter and microfin geometry. q
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.
Archives of Thermodynamics, 2000
Experimental investigations into the impact of the void fraction on the condensation characteristics of R134a refrigerant in minichannels under conditions of periodic instabilityThis paper present the results of experimental investigations of condensation of R134a refrigerant in pipe minichannels with internal diameters 0.64, 0.90, 1.40, 1.44, 1.92 and 3.30 mm subject to periodic pressure instabilities. It was established that as in conventional channels, the displacement velocity of the pressure instabilities distinctly depends on the frequency of their hydrodynamic generation. The void fraction distinctly influences the velocity of the pressure instabilities. The form of this relationship depends on the internal diameter of the minichannels and on the method of calculating the void fraction.
As microchannel heat exchangers have become more sophisticated in their design, more exact understanding of the flow inside them is necessary. A decrease in diameter enhances the heat transfer (which takes place at the inner walls of the tubes), but also increases the pressure drop (as the diameter decreases, it becomes like drinking a milkshake through a coffee stirrer). The inclusion of even small amounts of oil in circulation can have a significant effect as well. Historical correlations and studies of two-phase flow have been shown to be insufficient for predicting pressure drops in the smaller channels, due to the different fluid physics that are relevant in flows of small diameter. This study is aimed at understanding the fluid property effects that contribute to pressure drop and flow regime. Two-phase pressure drop data for four refrigerants (R134a, R410A, R290 and R717) were measured in a channel with hydraulic diameter of 148 ¹m. These data were combined with previous two-...
Micromachines
An analysis of the R134a (tetrafluoroetane) coolant’s non-stationary behavior in rectangular microchannels was conducted with the help of a newly proposed miniature refrigerating machine of our own design and construction. The experimental device incorporated, on the same plate, a condenser, a lamination tube and a vaporizer, all of which integrated rectangular microchannels. The size of the rectangular microchannels was determined by laser profilometry. R-134a coolant vapors were pressurized using a small ASPEN rotary compressor. Using the variable soft spheres (VSS) model, the mean free path, Knudsen and Reynolds numbers, as well as the dimensionless velocity profile can be assessed analytically. In order to determine the average dimensionless temperature drop in the vaporizer’s rectangular microchannels, in non-stationary regime, an analytical solution for incompressible flow with slip at the walls, fully developed flow and laminar regime was used, by aid of an integral transform...
This work presents a detailed characterization study of two-phase condensing flows of two refrigerants, R134a and R-245fa, in a single water-cooled micro-channel with a 0.4 mm x 2.8 mm cross-section (0.7 mm hydraulic diameter and 7:1 aspect ratio) and length of 190 mm. A single micro-channel was chosen for the study to eliminate flow mal-distribution issues commonly encountered in micro-channel banks. The experiments included a parametric study of the effects on average heat transfer coefficient and overall pressure drop of variations across the micro-channel condenser with mass fluxes between 50 and 500 kg/m 2 s, saturation temperatures between 30ºC and 70ºC, and inlet super heats between 0ºC and 20ºC. The inlet state was kept either at 100% quality or superheat and the outlet condition at 0% quality; these calculations were based on water-side heat rejection. Careful design and instrumentation of the test setup resulted in energy balance uncertainty within +/-11% and uncertainty of average heat transfer coefficient within +/-12%. It was determined that inlet superheat has little effect on heat transfer coefficient and overall pressure drop, but that the corresponding effects of saturation temperature and mass flux are significant.
A flow regime map for refrigerant condensation in herringbone micro-fin tubes (Part II).
Experiments were conducted with refrigerants R-22, R-134a and R-407C. Their thermodynamic properties were obtained from a reference database. 19 In these experiments, mass fluxes varied between 300 and 800 kg m -2 s -1 ; pressures between 1500 kPa and 1800 kPa; saturation temperatures between 39°C and 41°C; inlet vapour qualities between 0.85 and 0.98; and outlet vapour qualities between 0.02 and 0.15. Data were captured only once energy balance errors were below 1%. Uncertainties in the experimental work, which were calculated by Coetzee 20 and re-evaluated on the basis of present experimental results, 13 are presented in .
2017
Owing to the global warming effect consideration, the EU has banned the use of refrigerants with Global Warming Potential (GWP) value higher than 150 in all new passenger cars starting from 1 January 2017 (EU directive 2006/40/EC [1]. The study on the replacement of high GWP refrigerant such as HFC-134a has become an important and urgent issue. Recently, a new refrigerant, HFO-1234yf has been developed with similar thermodynamic properties to HFC-134a but much lower GWP value (GWP = 4 in comparing to 1,430 of HFC-134a). It is expected as a good candidate of replacing the refrigerant HFC-134a. However, most of the heat transfer and flow performances are still not very clear up-to-date. For reducing the weight of heat exchangers, most of the vehicles air-condition systems used extruded aluminum tubes in their evaporators and condensers. The hydraulic diameter of the heat exchangers is generally very small, which called microchannel heat exchanger, to resist the high working pressure o...
E3S Web of Conferences
The following paper presents the results of preliminary experimental research on the influence of instabilities of a hydrodynamic type on the condensation phase change process in tubular minichannels. The research was focused on a new pro-ecological refrigerant, R1234yf, intended as a substitute for R134a that currently is being phased out. The flow condensation phase change process was investigated for both steady and un-steady conditions in singular tubular minichannels with an internal diameter d = {1,44; 2,30; 3,30} mm. The scope of the analysis of the experimental data covered an estimation of propagation velocities for both pressure and temperature instabilities as well as the shrinkage of the condensation zone. The results were also compared with the previous results obtained for the flow condensation phase change of R134a refrigerant in tubular minichannels with the same internal diameters.