PHENOMENOLOGICAL ANALYSIS OF BOILING HEAT TRANSFER WITHIN THE LIQUID POOL OF A TWO-PHASE CLOSED THERMOSIPHON

Heat transfer studies of two-phase closed thermosiphons involve condensation in the cooling zones as well as boiling in the heating zones (pool boiling and liquid film boiling). The heat transfer coefficient of the condensation is usually calculated by using the classical theory of Nusselt or correlations based on this theory. Nevertheless there is not a reliable theory for predicting heat transfer coefficients applicable to pool boiling. In the past great efforts were taken to correlate experimental data. The first purpose of our study is to show analytically that the heat transfer coefficient in the case of pool boiling can be expressed in the form α ~ q2/3. In addition, the heat transfer coefficient depends on the pressure within the pool of the thermosiphon. The second purpose of our study is to deduce this pressure dependency from a heat transfer correlation applicable to forced convection boiling of water. Finally, the rate of heat transfer is limited. A lot of investigations deal with different limits of heat flux density submitted into the pool of the thermosiphon. According to the heat transfer coefficient, a corresponding limit of the wall superheating in the pool of the thermosiphon exists as well. Assuming a thermo mechanical effect of boiling, a mechanical non-equilibrium corresponds to the superheating of the wall. Therefore, the radial velocity of the vapour within the bubbles can be evaluated using a balance of momentum. The heat flux corresponding to the radial velocity of the vapour is limited by the speed of sound within the bubble. Conseqently, the third purpose of our study is to calculate analytically the limiting wall superheating when the velocity of the vapour equals the speed of sound.