The CO2 heat pump water heater ”ECO CUTE” which was commercialized in 2001 has a high potential for energy conservation and greenhouse abatement. The most important element apparatus is always the evaporator in order to develop smaller and higher performance equipment. In this paper, an experimental study has been conducted to measure the pure CO2 flow boiling heat transfer coefficient (99.999 % purity, without oil) in a horizontal smooth tube (outer diameter 6 mm, thickness 0.4 mm). The measured mean heat transfer coefficients are compared with calculated value with using previous experimental heat transfer correlation equations. These two values are different from each other. Mean heat transfer coefficients are measured with varying mass velocity, pressure and heat transfer lengths. The tube length is varied to 3.0 m, 4.0 m and 5.0 m, to distinguish the influence of mass velocity and that of heat flux to the heat transfer coefficient. The test conditions were: CO2 mass velocity from about 150 to about 700 kg⁄(m2s) (heat flux from about 10 to about 40 kW⁄m2), quality at inlet of test section is 0.17, CO2 super heat at outlet of test section is 5 K and saturation temperature of CO2 ranges from 0 to 10 °C. As a result, it has been understood that heat flux has a greater influence on the heat transfer coefficient.
Loop Heat Pipes (LHPs) are two-phase thermal control system, which works only by heat from its cooling target. In order to utilize the LHPs in various fields, it is requested to be smaller, more reliable, and higher performance. In the present study, a miniature LHP has been fabricated, and the effect of amount of working fluid charged on thermal performance of the LHPs has been investigated. Tests were conducted including start-up, power step up, as function of amount of working fluid. The test results showed that under-charging of working fluid caused start-up failure, while over-charging of working fluid made the LHP less stable.
Splashes are commonly seen when a solid object collides with liquid surface. It was recently found with macro-scale experiments that the surface wettability affects largely the splashing behavior; when its surface is water-repellent, the dropping object penetrates water with creating large air column (cavity) and generates large splashes. We examine similar situations with molecular dynamics simulation technique in order to compare the splashing behavior on nano scales with that on macro scales.
Effect of Prandtl number on three-dimensional mixed convection in a horizontal square duct with heated and cooled side walls was examined numerically. Non-dimensional governing equations were solved for Re = 100, Pr = 0.1˜10 and Ri = 36.44 by the SIMPLE method. Numerical results show that the spiral flow was generated along the flow direction, and its pitch lengthened with increase of Pr. Also, strength of spiral flow was evaluated by using Swirl number S. Corresponding to the flow, the heat transfer behavior was discussed. Heat transfer was promoted by the swirl flow with all Pr, and the optimum value existed within these Pr.