• International Journal of Air-Conditioning and Refrigeration
• /
• v.16 no.2
• /
• pp.51-63
• /
• 2008

A study of literatures on flow boiling heat transfer and two-phase flows inside microchannels is summarized. The potential applications, fabrication method and efforts to determine certain dimensional threshold for microchannels classifications are discussed. For the last two decades, numerous two-phase flow and heat transfer models for microchannels have been developed; many of them were derived from empirical models originally applied for conventional channels. Those models are discussed here along with a brief review on recent development of theoretical and phenomenological-based models for microchannels. This study is devoted to provide a review of important issues on flow boiling heat transfer and two-phase flows inside microchannels, including two-phase flow patterns, boiling heat transfer mechanism and correlations developments, pressure drop and prediction methods, and critical heat flux.

• Journal of Advanced Marine Engineering and Technology
• /
• v.29 no.8
• /
• pp.946-952
• /
• 2005

The $CO_{2}$ refrigeration cycle is expected to reduce the compressor work and increase the COP by applying two-phase ejector as a device for the recovery of dissipated expansion energy. In this study, the performance of the cycle was simulated and effects of the ejector shapes on the performance of the $CO_{2}$ refrigeration cycle were investigated. The following results were obtained through the cycle simulation. The COP of the $CO_{2}$ refrigeration cycle with two-phase ejector flow which expansion is occured in the isentropic manner is increased by a maximum of 24 $\%$ than the basic cycle with expansion valve If the velocity nonequilibrium in the mixing process is assumed the COP of the cycle is increased with the increase of the length and the decrease of the section area of the mixing tube. The best cycle performance is obtained when the divergent angle of diffuser is 7.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.10 no.4
• /
• pp.398-410
• /
• 1998

Two-phase flow phenomena are observed in many industrial facilities and make much importance of optimum design for nuclear power plant and the liquid transportation system. The particular flow pattern depends on the conditions of pressure, flow velocity, and channel geometry. However, the research on drag reduction in two-phase flow is not intensively investigated. Therefore, experimental investigations have been carried out to analyze the drag reduction and void fraction by polymer addition in the two-phase flow system. We find that the polymer solution changes the characteristic of two-phase flow. The peak position of local void friction moves from tile wall of the pipe to the center of the pipe when polymer concentration increase. And then we predict that it is closely related with the frau reduction.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.21 no.3
• /
• pp.140-148
• /
• 2009

Capillary tubes are widely used as expansion device in small refrigeration systems. The refrigerant flowing in the capillary tube experiences frictional and accelerational head losses and flashing simultaneously. In this paper flow characteristics of adiabatic capillary tubes were simulated with various friction factor models, two-phase viscosity models, and two-phase frictional multiplier models. The predicted pressure distribution and mass flow rate are compared with experimental data reported in literature. It is confirmed that the predicting accuracy with homogeneous model can be improved by employing suitable correlations of friction factor, two-phase viscosity and two-phase frictional multiplier.

• International Journal of Air-Conditioning and Refrigeration
• /
• v.14 no.1
• /
• pp.28-39
• /
• 2006

Two-phase flow through a T-junction has been studied by numerous researchers so far. The dividing characteristics of the gas and liquid phases at the T-junction are very complicated due to a lot of related variables. The prediction models have been suggested by using experimental data for a specific condition or working fluid. But, they showed the application limitation for the most of the other conditions or fluids. Since most of them are applicable for their own experimental range, the generalized model for the wide range of conditions and fluids is needed. Even though it's not available now, some of the models developed for air-water flow at a T-junction might be applicable for the part of refrigerants with some modifications. Especially, for the two-phase flow of refrigerants at the T-junction, very few studies have been performed. Further experimental study is required to be performed for the wide range of test conditions and fluids to predict properly the two-phase flow distribution and phase separation through the T-junction.

• International Journal of Air-Conditioning and Refrigeration
• /
• v.9 no.4
• /
• pp.27-36
• /
• 2001

A new two-phase non-boiling convective heat transfer correlation for turbulent flow $(Re_{SL}>4000)$ in vertical tubes with different fluid flow patterns and fluid combinations was developed using experimental data available from the literature. The correlation presented herein originates from a careful analysis of the major non-dimensional parameters affecting two-phase heat transfer. This model takes into account the appropriate contributions of both the liquid and gas phases using the respective cross-sectional areas occupied by the two phases. A total of 255 data points from three available studies (which included the four sets of data) were used to determine the curve-fitted constants in the improved correlation. The performance of the new correlation was compared with two-phase correlations from the literature, which were developed for specific fluid combinations.

• Proceedings of the SAREK Conference
• /
• 2008.06a
• /
• pp.255-262
• /
• 2008

The simulation of refrigeration cycle is important since the experimental approach is too costly and time-consuming. The present simulation focuses on the effect of capillary tube-suction line heat exchangers (CT-SLHX), which are widely used in small vapor compression refrigeration systems. The simulation of steady states is based on fundamental conservation equations of mass and energy. These equations are solved simultaneously through iterative process. The non-adiabatic capillary tube model is based on homogeneous two-phase model. This model is used to understand the refrigerant flow behavior inside the non-adiabatic capillary tubes. The simulation results show that both of the location and length of heat exchange section influence the coefficient of performance (COP). These results can be used in either design calculation of capillary tube length for refrigeration cycle or effect of suction line heat exchanging on refrigeration cycle.

• International Journal of Air-Conditioning and Refrigeration
• /
• v.11 no.4
• /
• pp.159-169
• /
• 2003

Five two-phase closed loop thermosyphons (TLTs) specially designed and constructed for the present study are one small scale loop, two medium scale loops (MSLI and MSLII) and two large scale loops (LSLI and LSLII). Two simulation models based on thermal resistance network, lumped and sectorial, are presented. In the Lumped model, the evaporator section is dealt as one lumped boiling section. Whereas, in the Sectorial model, all possible phenomena which would occur in the evaporator section due to the two-phase boiling process are considered in detail. Flow regimes, the flow transitions between flow regimes and other two-phase parameters involved in two-phase flows are carefully analyzed. In the present study, the results of two different simulation models are compared with experimental results. The comparisons showed that the simulation results by the Lumped model and by the Sectorial model did not show any partiality for the model used for the simulation. The simulation results according to the correlations show the various results in the large different range.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.18 no.1
• /
• pp.31-37
• /
• 2006

The present study has been experimentally investigated the effect of geometric and operating parameters on the two-phase flow distribution of refrigerants in a horizontal T-junction. The operating parameters were the kind of refrigerants (R-22, R- l34a, and R-410A), saturated temperature, and the inlet mass flux and quality. The geometric parameters were the tube diameter and the tube diameter ratio. The measured data of refrigerants were compared with the values predicted using the models developed by several researchers for air/water or steani/water two-phase flow. Among the operating parameters, the inlet Quality was the most sensitive to the mass flow rate ratio. Between the geometric parameters, the tube diameter ratio was more sensitive than tube diameter.

• Journal of Advanced Marine Engineering and Technology
• /
• v.28 no.8
• /
• pp.1195-1202
• /
• 2004

In this paper an experimental study was investigated for two-phase flow distribution in compact heat exchanger header. A test section was consisted of the horizontal bottom dividing header($\phi$: 5 mm, L: 80 mm) and 10 upward circular mini channels ($\phi$: 1.5 mm, L: 850 mm) using an acrylic tube. Three different types of tube intrusion depth were tested for the mass flux and inlet mass quality ranges of 50 - 200 kg/$m^2$s and 0.1 - 0.3, respectively. Air and water were used as the test fluids. The distribution of vapor and liquid is obtained by measurement of the total mass flow rate and the calculation of the quality. Two-phase flow pattern was observed, and pressure drop of each channel was measured. By adjusting the intrusion depth of each channel an uniform liquid flow distribution through the each channel was able to solve the mal-distribution problem.