Kohei Koyama

Effect of Partition Wall on Heat Transfer Characteristics of a Gas-to-Gas Counterflow Microchannel Heat Exchanger

The effect of a partition wall on heat transfer characteristics of a two-stream gas-to-gas counterflow microchannel heat exchanger has been numerically investigated. The flow passages of the microchannel heat exchanger are plane channels of 100 μm in height and 20 mm in length. The material of the partition wall is assumed to be stainless steel. The computations were performed for a wide range of flow rate to investigate heat transfer characteristics of the microchannel heat exchanger. Moreover, computations for various partition wall thicknesses were conducted to investigate the effect of the wall thickness. The thickness ranged from 200 μm to 6 μm while the channel height was fixed at 100 μm. Numerical results show that heat transfer characteristics of a gas-to-gas counterflow microchannel heat exchanger are affected by partition wall thickness. Computations for various partition wall thicknesses and thermal conductivities of the partition wall were performed. The results were compared with those of a single microchannel with constant wall temperature. Applicability of the assumption of constant wall temperature was revealed.

Visualization of FC-72 Flow Boiling in Parallel- and Counter-Flow Plate Heat Exchangers

This study investigates FC-72 (Perfluorohexane) flow boiling in a plate heat exchanger. A plate heat exchanger which has a transparent cover plate is manufactured to visualize boiling two-phase flow pattern of the working fluid FC-72 heated by hot water. Titanium is used for heat transfer plate, which has micro pin-fin structure on the heat transfer surface to enhance heat transfer. Experiment is conducted for parallel- and counter-flow arrangements to compare thermal and hydraulic performances. Flow boiling is photographed by a digital camera and instantaneous images are processed to classify flow pattern and to measure void fraction in the heat exchanger. Flow rates and temperatures of FC-72 and hot water at inlet and outlet of the heat exchanger are simultaneously measured to obtain overall heat transfer coefficient. Two-phase flow pattern of FC-72 flow boiling and void fraction along flow direction as well as thermal performance are discussed. Experimental results show that bubbly flow, slug flow, and churn flow are observed for the experimental range of this study. Extent of churn flow in the parallel-flow heat exchanger is larger than that of the counter-flow one due to generated bubbles at upstream region in working fluid channel. Void fraction of the parallel-flow plate heat exchanger increases rapidly compared with that of the counter-flow one due to location of onset of nucleate boiling. Overall heat transfer coefficients for the parallel-flow arrangement is larger than that of the counter-flow due to destruction of thermal boundary layer. The experimental results show that flow arrangement of a plate heat exchanger has the potential to improve its thermal performance.Copyright

Effect of Channel Geometry on Ammonia Boiling Heat Transfer of a Plate-Type Evaporator

The ocean thermal energy conversion (OTEC) is attracted attention as one of the promising renewable energy. OTEC uses small temperature difference between surface and deep sea water. Plate-type heat exchangers, or evaporators, are usually used for OTEC to obtain vapor for electric generator. Ammonia is used for OTEC as a working fluid. It is important to improve thermal performance of an evaporator for the OTEC. Channel dimension is one of the important factors to improve heat transfer performance of an evaporator. In this study, the measurement and comparison of local heat transfer coefficient for three channels are experimentally performed. The experiments are conducted for a range of mass flux (5 and 7.5 kg/m2s), heat flux (10 to 25 kW/m2), and pressure (0.7 and 0.9 MPa). The results show that the heat transfer coefficient increases as decreases channel height. The modified emprical correlation for a plate-type evaporator is proposed.Copyright

Availability of a high polymer resin coating aluminum to the plate heat exchanger for ocean thermal energy conversion plant using ammonia as working fluid

Ocean thermal energy conversion(OTEC)has attracted attention as a technique to obtain renewable energy. OTEC systems usually use a plate heat exchanger and ammonia as a working fluid. The materials used for manufacturing the heat exchanger are generally “titanium” or “stainless steel” of which contact surface should not be corroded with ammonia. However, the thermal conductivity of these materials is very low. Consequently, the heat transfer performance of the heat exchanger deteriorates. Therefore, the author proposed an advanced plate heat exchanger material with high thermal conductivity for OTEC systems. This plate is made of an aluminum alloy and the surface is coated with a special high polymer material(PEEKTM polymer), which has high ammonia resistance. In this study, two tolerance experiments were performed using the advanced plate at different coating thicknesses of 300 and 20 μm for approximately one month. These experiments were(1)immersion of the plate in a pressure tank containing liquid ammonia and(2)exposure of the plate in the path of a forced convective flow of liquid ammonia. As a result, although the PEEK coating aluminum plate surface deteriorated slightly, it is found that the base of the plate has not influenced by the ammonia. (Received August 24, 2012 Accepted January 18, 2013)

A Constant-Wall-Temperature Model for Prediction of Thermal Performance of Gas-to-Gas Counter-Flow and Parallel-Flow Microchannel Heat Exchangers

Thermal performances of gas-to-gas counter-flow and parallel-flow microchannel heat exchanger have been investigated. Working fluid used is air. Heat transfer rates of both heat exchangers are compared with those calculated by a conventional log-mean temperature difference method. The results show that the log-mean temperature difference method can be employed to a parallel-flow configuration whereas that cannot be employed to a counter-flow configuration. This study focuses on the partition wall which separates hot and cold passages of the microchannel heat exchanger. The partition wall is negligibly thin for a conventional-sized heat exchanger. In contrast, the partition wall is thick compared with channel dimensions for a microchannel heat exchanger. A model which includes the effect of the thick partition wall is proposed to predict thermal performances of the microchannel heat exchangers. The heat transfer rates obtained by the model agree well with those obtained by the experiments. Thermal performances of the counter-flow and parallel-flow microchannel heat exchangers are compared with respect to one another based on temperature of the partition wall. The comparison results show that thermal performances of the counter-flow and parallel-flow microchannel heat exchangers are identical. This is due to performance degradation induced by the thick partition wall of the counter-flow microchannel heat exchanger. This study reveals that the thick partition wall dominates thermal performance of a gas-to-gas microchannel heat exchanger.Copyright