Experimental Analysis of Pressure Drop in Single and Two Phase in Mini Channels (original) (raw)
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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-...
IOP Conference Series: Materials Science and Engineering, 2015
This study is a computer simulation of the temperature profiles and experimental investigation of three 100 mm x 50 mm x 18 mm single mini channel condensers with hydraulic diameters of 3 mm, 2 mm, and 1mm. The mini channels which were made of copper were designed, fabricated and tested. Each unit was connected in a vapor compression cycle with R-134a as the refrigerant. The average refrigerant mass flow rates were varied from 1.296-69.471 g/s, and the average inlet and outlet condenser pressure variations were 102.5-121.8 kPa and 101.74-121.23 kPa, respectively. Each condenser was placed inside a mini wind tunnel system where forced draft air was introduced to initiate convective heat transfer. Each condenser was tested and data were gathered every five minute interval for one hour using a Lab View Software. Computer simulations on the flow process were conducted using Solid Works software. The experimental results presented the inlet and outlet condenser pressures, and pressure drops. The experimental heat transfer coefficients were calculated at different mass fluxes during condensation. The values ranged from 3900 to 5200 W/m 2-°K for the 3 mm, 2600 to 9000 W/m 2-°K for the 2 mm, and 13 to 98 W/m 2-°K for the 1 mm. The heat transfer coefficients calculated from experiments were then compared with the computed values using the correlations developed by Dittus-Boelter and Lee-Son. The results showed increasing deviation as the diameter decreased. The discrepancies could be attributed to the appropriateness of the Dittus-Boelter and Lee-Son correlations in small diameter channels, complexities in the flow process which involved two phase flow heat transfer in very small tubes, and the difficulties in attaining very accurate measurements in small channels.
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
1999
This paper presents a pressure drop correlation for evaporation and condensation in smooth and micro-fin tubes for lubricant-free refrigerants and refi-igerantllubricant mixtures. The form of the generalized correlation was taken from the Pierre pressure drop model. NIST micro-fin tube pressure drop data for R134a, R22, R125, R32, R407C, R41OA, and R32R134a (25175 % mass) were regressed to a modified Pierre correlation. The NIST database was post-predicted with an average absolute residual of 10.8 %. Further validations performed with extensive data from the literature for lubricant-free refhgerants in smooth and micro-fin tubes showed an average absolute residual between measurements and predictions not to exceed 17.6 % for the various data sets. The condensation and evaporation pressure drops for different refrigerantllubricant mixtures were predicted with average absolute residuals not exceeding 19.6 % and 28.0 %, respectively.
This paper describes modeling of two phase flow distribution in a microchannel evaporator, on the basis of the pseudo 2-D finite volume method. Emphasis is placed on refrigerant-side heat transfer and pressure drop characteristics. The global flow distribution is based on the mechanistic fact that the pressure drop along each flow path containing an individual microchannel tube must be the same. Both the primary pressure drop across the microchannel tube and minor pressure drops are included.
Air-side heat transfer and pressure drop characteristics of accelerated flow evaporators
International Journal of Refrigeration, 2011
Microchannels are likely to be the next heat transfer technology for household refrigerating applications, especially due to their compact design and high heat transfer rate per unit of volume. In contrast to the conventional tube-fin evaporators, the available heat transfer and pressure drop correlations are not well-stablished yet. This study introduces a new microchannel evaporator design to be used in 'nofrost' household refrigerators. The performance of sixteen evaporators prototypes with distinct geometric characteristics were evaluated experimentally in an open-loop wind-tunnel calorimeter facility coming up with data for the overall thermal conductance and the air-side pressure drop data as a function of the air flow rate. Empirical correlations for Colburn j-factor and friction f-factor were devised in terms of the Reynolds number and key compact heat exchanger parameters, being able to predict 90% of the experimental counterparts within ±10% and ±20% error bands, respectively. Furthermore, comparisons against a typical no-frost tube-fin evaporator was carried out with respect to air-side pressure drop and overall thermal conductance, which indicated a promising potentiality of application in household refrigeration appliances.
Effect of Liquid Reynolds Number on Pressure Drop of Evaporative R-290 in 500µm Circular Tube
International Journal of Technology
Due to certain advantages, natural refrigerants have recently become more popular. Environmental issues motivate this study, focused on the characteristics of propane (R-290) as a replacement for conventional refrigerants. The aim of the present research is to characterize the pressure drop of evaporative R-290 in a microchannel of 500µm diameter and 0.5 m length. The variables of the experimental conditions are mass flux between 155 and 1071 kg/m 2 s and vapor quality between 0 and unity. The results show a laminar flow for liquid R-290 and a turbulence flow for vapor. Some existing correlations of two-phase flow viscosity were used to predict the pressure drop. For homogeneous model, Dukler et al.'s (1964) prediction viscosity correlation best predicted the present experimental pressure drop.
Refrigerant 134a liquid flow through micro-scale short tube orifices with/without phase change
Experimental Thermal and Fluid Science, 2006
The paper presents the results of experiments when R134a was flowing through micro-orifices with diameters of 31.0 and 52.0 lm, and length-to-diameter ratio of 2.5 and 4.2, respectively. For liquid-upstream/liquid-downstream flow, the discharge coefficient was found to be independent of Reynolds number, which suggests separated flow that was defined in macro-scale orifices. For liquid-upstream/two-phase-downstream flow, the experimental results indicate significant departure of flow characteristics from macro-scale orifices. The flow was not choked even when the downstream pressure was reduced to more than 400.0 kPa below the saturation pressure corresponding to upstream temperature, whereas in normal size orifices with length-to-diameter ratio larger than two, the flow is typically choked as downstream pressure is reduced below the saturation pressure. This phenomenon was explained by the strengthening of metastable effect in micro-tubes. For liquid-upstream/two-phase-downstream flow, a semi-empirical correlation was developed by modifying the discharge coefficient of the conventional orifice equation.