Study Of Cryogenic Fluid’s Pool Boiling Mechanism And CFD Simulation Of Its Forced Flow Cooling System

Cryogenic fluid, which consists of liquid helium, liquid nitrogen and liquid hydrogenis generally employed as working medium in refrigeration loop or condensation process. Itworks effectively to preserve a constant temperature for the systems equipped withtechnologies of nuclear magnetic resonance, superconductivity and forward lookinginfrared imaging. Pool boiling phenomenon of cryogenic liquid on smooth surface andfoam surface is of great significant in tems of phase change heat transfer. Deepinvestigation on pool boiling heat transfer mechanism of cryogenic fluid extremelycontributes to acquire intrinsic correlation between surface heat flux, surfacetemperature difference and boiling heat transfer coefficient. This thesis accomplishesexperimental study of cryogenic fluid’s pool boiling heat transfer mechanism and CFDsimulation of heat load of its forced flow cooling system, and the main content is asfollow:First, introduced the design of the entire experimental system for liquid nitrogen’spool boiling on different types of surface, operation procedures of the experiment andanalysis of systematic errors in the experiment. Since the experimental system waspreserved in liquid nitrogen, hexagon-head bolts and spring shim were used to fasten theflanges, in order to reduce the heat leakage. Besides, due to Teflon’s contraction effect atlow temperature, gasket made of teflon was strongly squeezed with a rather severedeformation to insulate chamber from liquid nitrogen. CFD simulation was utilized totestify the one-dimensional conduction hypothesis of the copper block and accomplish theanalysis of heat leakage.The correlation curves of surface heat flux-surface temperature difference, boilingheat transfer coefficient-surface heat flux were obtained on smooth copper surface andfoam copper surface respectively. The experimental result on smooth surface finely provedthe typical boiling curve. CHF was observed at heat flux of17.3W/cm2, meanwhile the temperature difference was14.65K. While the result on foam surface obviously disagreedwith the typical boiling curve. At the beginning of nucleate boiling, the boiling heattransfer coefficient was large as0.65W/(m2·K). When surface heat flux surpassed2.75W/cm2, the boiling heat transfer coefficient reduced to0.3W/cm2.Nucleate boiling regime on foam copper surface was divided into two distinct boilingregimes: Base boiling and Surface boiling. In Base boiling regime, isolated bubbles weregenerated inside the pores of the foam material. And the bubble became largeraccompanied by the increasing of surface heat flux. In Surface boiling regime, liquid waspushed out of the pores coatings of the foam material by superheated vapor, and boilingonly took place at the surface of the coating. With the encounter-effect between upsidevapor and downside cold liquid, the "mushroom cloud" structure was formed and steadyvapor film was seldom observed on the surface. If the boiling process on smooth suafacehad surpassed the critical point, hysteresis phenomenon would occur when decrease theheat flux. Hysteresis phenomenon would disappear as heat flux reduced to2.745W/cm2.Hysteresis phenomenon on foam surface never appeared since the boiling process had notstepped into film boiling regime.According to the visualization of pool boiling, boiling state on different surface wasdivided into five regimes with the increasing of heat flux:1. Transition from naturalconvection to nucleate boiling,2. Nucleate boiling developing regimeⅠ(Base boilingregime on foam surface),3. Nucleate boiling developing regimeⅡ(Transition from Basesurface to Surface boiling on foam surface),4. Nucleate boiling developing regime Ⅲ(Gas column appeared on the above of smooth surface and gradually reached critical point;Surface boiling on foam surface),5. Film boiling regime.CFD simulation of of heat load of cryogenic fluid forced flow cooling system wasaccomplished. Half model was built and DO method was employed to calculate radiationheat flux in complicated system. Temperature distribution of high vacuum chamber andbeam plates was obtained. Method of coupling forced flow and cryocooler cooling systemwas applied to guarantee beam plates under critical temperature with superconductivity.