دانلود رایگان مقاله متدلوژی ساختار منحنی جوش برای خنک سازی اسپری کولر

1. Introduction

Film boiling is encountered in various applications like metallurgy, refrigeration, chemical and power engineering, etc. In particular knowledge of film boiling properties of coolants is of importance for nuclear reactors to assess the safety of nuclear reactor designs. Generally experimental boiling curves are obtained only until critical heat flux (CHF) as beyond this heat flux there is a vapor blanket on the surface and there is a thermal runaway situation. So conventional method of experimentation fails in the regime beyond CHF. Conventional experimental set up [1] involves controlled heat flux where heat flux is slowly increased and a stable steady state is obtained for each heat flux. But in such a set up beyond CHF, there is indefinite temperature rise. Another alternative is to have temperature controlled system [2], which would need a pressurized two phase liquid boiler to produce different temperatures. Such a system is complicated and expensive. Previous studies or reports in literature [3–10] concern mainly with extraction of boiling curves for pool boiling. Full boiling curve of saturated methanol was reported [3] for horizontal cylinders under pool boiling conditions. The possibility of cooling a superconducting magnet using liquid nitrogen, led to a report [4] of forced convection heat transfer study of liquid nitrogen inside horizontal tubes. But the study was limited to near nuclear boiling regime. Transient study of pool boiling of liquid hydrogen from a flat surface was reported [5] but limited until the critical heat flux. Numerical simulation and experiments of boiling of various cryogenic liquids were studied [6] to understand the pool boiling mechanism. Transition boiling and film boiling of FC-72 from horizontal cylinders and wires under pool boiling conditions were reported in [7]. Effect of heater orientation in liquid nitrogen pool boiling was reported in [8]. The optimal geometry of tube bundle for nucleate boiling of liquid nitrogen (over a tube bundle) was studied and reported in [9]. All regimes – nucleate boiling, transition and film boiling were reported in [10] for liquid nitrogen pool boiling over a thin wire. In summary there has been numerous studies of different cryogenic fluids under pool boiling conditions.

7. Results and discussion The proposed solution to solve this problem is described in this section. The effect of actual heat loss/gain from the copper block to the adjoining heater structure, insulation and surroundings is to reduce/increase the surface temperature for a given heat flux and spray conditions. If a large copper block with no heat loss is modelled in the inverse problem being solved it will result in increase/ decrease of transient surface temperature (compared to actual experimental conditions) during cooling/heating. Since the total measurement time is short (200–500 s), it is a valid assumption to model a slightly longer copper block, whose extra thermal mass will take into account the transient heat loss/gain. However if the assumed size is smaller compared to the actual equivalent size, then the heat transfer coefficients during transient heating will be higher than the actual value. This means that for an applied heat flux and measured temperature, the model would represent that less energy is used up for self-heating and large amount of heat is removed from the top boundary. Similarly during a cooling curve, during which heat supply is turned off, for a measured temperature drop, a smaller size of the equivalent copper block length would mean that the amount of heat flux removed is lower and would result in lower heat transfer coefficients. So a smaller assumed length would result in higher heat transfer coefficients during transient heating and lower values during transient cooling. However the heat transfer coefficient is expected to be the same at a given surface temperature for the same flow conditions. So the length is varied until the heat transfer coeffi- cients for heating and cooling converge. It should be noted that it is assumed that the heat transfer is more prominently one dimensional and the lateral conduction will not significantly affect the inverse heat conduction problem solved to estimate the transient heat flux removed at the top boundary.