Tadakatsu Nakajima

Effects of Pore Diameters and System Pressure on Saturated Pool Nucleate Boiling Heat Transfer From Porous Surfaces

The porous surface structure was manufactured with precision for the experimental study of nucleate boiling heat transfer in R-11. Boiling curves and the data of bubble formation were obtained with a variety of geometrical and operational parameters; the pore diameters were of 50, 100, 150 ..mu..m, there was a combination of pores of different sizes; and the system pressures were of 0.04, 0.1, 0.23 MPa. The boiling curves exhibit certain trends effected by the diameter and population density of pores. A combination of high system pressure and pore sizes of 100 or 150 ..mu..m dia enables boiling to persist even when the wall superheat is reduced to an extremely low level of 0.1 K. A noteworthy feature of porous surface boiling is that intense bubble formation does not necessarily yield a high heat-transfer performance. Examination of the data indicates that liquid suction and evaporation inside the cavities are a proable mechanism of boiling with small temperature differences.

Enhancement of a two-phase thermosyphon for cooling high heat flux power devices

The purpose of this study is the enhancement of cooling of high heat flux power devices such as a thyristor by a thermosyphon system. The thermosyphon uses boiling and condensation of an inert dielectric fluorocarbon (FC-72). Boiling occurs from a multiple chimney heat transfer structure. A boiling chamber is connected to the condenser by a double tube, with the inner tube carrying the condensed liquid and the outer annulus carrying vapor. Using the experimental model of a thermosyphon, heat flux capacity is enhanced by increasing the flow tube height H/sub 0/, boiling chamber height H/sub h/, and condenser liquid level H/sub l/. High heat flux capacity is achieved with H/sub 0/>8 cm, H/sub h/>10 cm, and H/sub l/>H/sub 0/, using the chimney heat transfer structure with a channel width of 3 mm. The thermosyphon system is able to cool a maximum heat flux of 3 kW per device. >

A Design for Loop Thermosyphon Including Effect of Non-Condensable Gas

We have developed a loop thermosyphon for cooling electronics devices. Its cooling performance changes with the ambient temperature and amount of input heating. Especially it deteriorates with non-condensable gas (NCG) increase. NCG leakage of thermosyphon cannot detect below under 10−10 Pa-m3 /s, though we have to design the thermosyphon considering these characteristics to provide guaranteed performance for 5–10 years. In this study, the effect of the amount of NCG in each component of a thermosyphon was measured while changing the amount of heater input, and the amount of NCG. As a result, we obtained some useful design information. The performance of air cooling part does not depend on the NCG amount in this case. The performance of evaporation part depends on the total pressure that includes the partial pressure of vapor and the partial pressure of NCG. The performance of condensation part is deteriorated strongly by NCG amount increase. Additionally, we expressed these performances as approximations. These expressions let us predict the total thermal resistance of this thermosyphon by the NCG amount and the input heating amount. Then, using the leakage of a thermosyphon and the amount of dissolved NCG in water, we predicted the amount of NCG that will be in the thermosyphon after 10 years. These results also let us predict the thermosyphon’s total thermal resistance after 10 years. Though there is a slight leakage on thermosyphon, using this technique, we are able to design a thermosyphon that is guaranteed the cooling performance for a long term.Copyright

Distribution of heat transfer coefficients from small surfaces cooled with submerged jets of fluorocarbon liquid determined by inverse analysis of heat conduction

Convective boiling from small surfaces with submerged impingement of fluorocarbon liquid is investigated for high-performance cooling systems of electronic devices. The local heat transfer coefficients from silicon heaters in submerged impingement are measured by two independent techniques. The silicon heaters used for this measurement are constructed of many cells which can independently generate heat on one side of the heater and are cooled on the other side. Each cell also contains a sensor to measure the temperature, One of the independent techniques we use is to control the distribution of the heater power to produce a uniform temperature. With this technique, the variation of the heat flux distribution between each side of the heater caused by the heat conduction can be eliminated and the heat transfer coefficients measured more accurately. The second technique we use is inverse analysis. Inverse analysis is used to correct the effect of heat conduction in the heater and to determine the distribution of heat transfer coefficients on the cooling surface. The distribution of heat transfer coefficients determined with these techniques agreed well with the equations used to describe the convective boiling.