Monde and co-workers [3 - 7] published some papers concerned with surface temperature and heat flux during quenching a high temperature material by an immersion of pool liquid, impinging jet and falling liquid film. These values are estimated from measured temperatures in the material using their proposed inverse solutions [5 – 7]. Recently, Monde et al. [8] noticed that the previous values are estimated lower than actual ones for the case that a surface heat flux quickly changes in a time lower than 1 s. In addition, those estimated values are found to strongly depend on thermal properties of the material. The previous estimated surface temperature and heat flux are firstly reexamined based on the recent result proposed by Monde et al. [8]. The present study finally pointed out that the previous maximum heat flux during a cooling of each cooling system from inverse solution for heat conduction should be corrected to be correspond to the critical heat flux. A trigger which makes the transition from film boiling to nucleate boiling is that the surface heat flux during the cooling of high temperature solid reaches the corresponding critical heat flux.

Monde and Mitsutake [1] recently proposed a new idea for immersion cooling that the quench starts when the surface heat flux q(t) during the immersion cooling reaches exactly the critical heat flux q_c at a critical time t_c* depending on a couple of solid and liquid. This time t_c* during immersion cooling of liquid nitrogen (LN₂) appears t_c* = 1.4 ~ 2.4 ms for some rare sphere solids. Monde and Mitsutake [2] expand their idea for the immersion cooling of Al sphere coated by a thin Teflon film and explain the reason that the temperature at which its quench occurs, shifts to higher temperature with increase in the thickness of Teflon. This study firstly focuses on the short period of time immediately after the immersion of a cylinder coated by thin film into a liquid and then derives an exact solution for the temperature during the immersion cooling. It reveals for about 10 ms after a contact of the Cu cylinder coated by Stycast with liquid Nitrogen(LN₂) that the temperature for the cylinder is uniformly kept at a constant temperature, while the surface temperature on Stycast contact with LN₂ drops at a moment of their contact until a certain temperature which is determined by both initial temperatures of the cylinder and LN₂ before their contact and then the temperature in Stycast film largely changes with time. In addition, the quench starts within the time of t_c* = 10 ms, after the immersion of the Cu cylinder coated by Stycast into LN₂．The mechanism that the coated film makes the temperature, at which the quench starts, higher, is clearly explained.