廣川 智己

Herein, the heat transfer characteristics of a shear-driven liquid film flow, which is a liquid film flow with co-current gas flow, in a rectangular channel were experimentally investigated. An experimental apparatus was fabricated to observe the liquid film behavior and evaluate the local heat transfer coefficient. The rectangular channel had a height of 2 mm and a width of 10 mm, and its heating surface was a horizontal flat plate with 10 mm in width and 100 mm in length. The experiments were conducted by varying the heat flux and gas Reynolds number by maintaining the liquid film Reynolds number constant. Based on the experimental results, the liquid film behavior was classified into six patterns: bubble flow, annular flow, annular flow with drying and wetting, annular flow with rivulet, stratified flow, and annular flow with dry area. Although the local heat transfer coefficient in the shear-driven liquid film flow decreased due to the expansion of dry area caused by the rupture of liquid film compared with that of flow boiling, the local heat transfer coefficient did not decrease considerably. This indicated that considerably high heat flux occurred in the wet area of the rivulet flow due to decreased liquid film thickness.

Abstract This paper presents an experimental investigation of local heat transfer characteristics of single-phase flow in a plate heat exchanger (PHE). The local heat transfer coefficient is evaluated using a test section with PHE geometry for measuring wall temperature distribution. The test section of 1.5 mm thickness is employed to consider the heat conduction effect of the heat transfer plate. The results indicated that the local heat transfer coefficient is influenced by the development of the thermal boundary layer along the flow direction and the maldistribution of water flows along both the direction perpendicular to the flow and the stacking direction. The harmonic mean heat transfer coefficient calculated by the measured local heat transfer coefficient agrees with the average heat transfer coefficient evaluated by the modified Wilson plot method within ±25% and within ±16% for the hot side and the cold side, respectively.

Heat exchanger modeling has been widely employed in recent years for performance calculation, design optimizations, real-time simulations for control analysis, as well as transient performance predictions. Among these applications, the model’s computational speed and robustness are of great interest, particularly for the purpose of optimization studies. Machine learning models built upon experimental or numerical data can contribute to improving the state-of-the-art simulation approaches, provided careful consideration is given to algorithm selection and implementation, to the quality of the database, and to the input parameters and variables. This comprehensive review covers machine learning methods applied to heat exchanger applications in the last 8 years. The reviews are generally categorized based on the types of heat exchangers and also consider common factors of concern, such as fouling, thermodynamic properties, and flow regimes. In addition, the limitations of machine learning methods for heat exchanger modeling and potential solutions are discussed, along with an analysis of emerging trends. As a regression classification tool, machine learning is an attractive data-driven method to estimate heat exchanger parameters, showing a promising prediction capability. Based on this review article, researchers can choose appropriate models for analyzing and improving heat exchanger modeling.

Recently, air-to-refrigerant micro-channel heat exchangers have been applied to air-conditioners for their high heat-transfer performance with low air-side pressure drop. Some papers have presented the experimental data on microchannel flat tube applied to such heat exchangers. Among these papers, however, uniform heat load is applied to the test tube unlike a real situation. In the real use, heat load becomes un-uniform along air flow direction as air temperature changes during heat exchange. In this paper, the effect of un-uniform heat load on heat transfer is experimentally investigated. Measurements were carried out by heating microchannel flat tube flowing R32 horizontally. Un-uniform heating is achieved by two each heating blocks with different temperature attached to the test tube from top and bottom. It is found that un-uniform heat load degrades heat transfer comparing to uniform load and it becomes clear that understanding detail phenomena are necessary to predict heat exchanger accurately.

A micro-channel heat exchanger has high performance of heat transfer compared with a conventional finned tube heat exchanger. However, there are some challenges when it applied to a heat pump system. One of the most difficulties is a refrigerant mal-distribution when it is used as an evaporator. Especially, when the tubes are arranged parallel to vertical direction, distributing the two-phase refrigerant flow to each tube equally is a challenge because of the density difference between liquid and vapor. Moreover, the tendency of mal-distribution is changed depending on the mass flow rates at each operating conditions. To solve these problems, a new configuration of header named “Refrigerant loop header.” is proposed. It enables to distribute refrigerant equally to each tube corresponded to mass flow changes.

Experiments were performed to verify the performance of experimental apparatus for the acquisition of reference data for flow boiling heat transfer under the terrestrial condition which is to be compared with that obtained under the microgravity condition onboard International Space Station (ISS) by using another apparatus with the same specification. Test section is a circular tube made of copper with an inner diameter of 4 mm and a heated length of 368 mm and oriented vertically on ground. To improve the accuracy of local heat fluxes, the compensation of heat flux distribution along the tube axis is discussed on the basis of the experimental results on the local heat transfer coefficients for a single-phase liquid flow. Correlations for local heat transfer coefficient of flow boiling are proposed here as functions of boiling number and Martinelli parameter in the regions of nucleate boiling and two-phase forced convection, respectively. Because the discrepancy of local heat transfer coefficient obtained from the apparatus for the terrestrial and the space experiments is caused by the difference of surface roughness in nucleate boiling region, a compensation factor is introduced in the correlation. The local heat transfer coefficients predicted by the proposed correlation are agreed well with those obtained by both apparatus.