Phase Change Heat Transfer Performance On Porous Coatings And An Application-Vapor Chamber

Applications of porous coatings to enhance heat transfer and corresponding manufacturing processes are commonly encountered in industries, agricultures, military, biomedicine technologies and many other high and new technologies. Meanwhile, the phase change phenomena occurring in thin porous coatings have many unusual characteristics due to their special structures. In order to solve the problems encountered in practice and better utilizing function of the structures such as porous coating to enhance heat transfer, it is highly necessary to conduct associated fundamental investigations for thorough insights into the phase change phenomena in porous media and corresponding physical mechanisms.In this investigation, through parallel experiments conducted on porous coatings and plain surface, an attempt was made for comprehensive understandings of the boiling regimes and heat transfer mechanisms. The pool boiling on porous coating displayed some specific characteristics and its importance in heat transfer enhancement in either Natural Convection accompanied by the initiate nucleation or Individual Bubble boiling increasing in population or Vapor-liquid Counterflow Boiling with violent motion of two phase flow. Porous coatings can enhance heat transfer in many aspects including: decrement in surface superheat, enhancement in heat transfer coefficient and CHF, and uniforming the surface temperature, and the mechanisms involve increasing nucleate sites density, strengthening the shear stress between two phases, efficiency of capillary pumping, reducing critical length and are closely dependent of wick structure and many other conditions.As an important application-Vapor Chamber has many advantageous, such as high equivalent thermal conductivity, uniform temperature of bottom of evaporator, lightening coolers, and so forth all of which contribute to the cooling capacity. When a different heat flux was applied, heat transfer processes in both evaporating and condensing plates or sides of Vapor Chamber showed different Characteristics. When the heat flux was below the onset of nucleate boiling, fluid in evaporator only evaporated on the vapor-liquid interface, and the liquid film on surface of condenser was stable and had a uniform thickness. When the heat flux was increased, strong vapor-liquid counterflows or tiny bubbles injecting vapor stream occurred accompanied with various liquid film thickness and large drops on surface of condenser. The thermal resistant between and evaporator and condenser of the Vapor Chamber decreased when the heat flux increased. This observation confirms that the onset of nucleate boiling in porous wicks does not mean the heat transfer limitation is reached. On the contrary the heat transfer was improved by the Vapor-liquid Counterflow Boiling. Means as utilizing porous wick of high merits and integrated performance, reducing the height of vapor chamber and charging with proper rate can greatly improve the performance of Vapor Chamber in practice.