Transport phenomena associated with phase change in homogeneous and inhomogeneous systems

Shankar Krishnan, Purdue University

Abstract

Metal foams and solid-liquid phase change materials (PCMs) are investigated for application in thermal management. Two different passive heat sink designs using PCMs are proposed for transient thermal control of electronics. In the first of the two designs, PCMs are proposed as an alternative to copper heat sinks used to absorb high-power thermal transients in power semiconductors. A novel hybrid heat sink concept is proposed for thermal management applications that encounter time-dependent cooling conditions. The concept involves a plate-tin heat sink with the tip immersed in a PCM. The exposed area of the fins dissipates heat during periods when high convective cooling is available. When the air cooling is reduced, the heat is absorbed into the PCM. Besides rigorous numerical calculations, easy-to-use expressions and design guidelines are developed for both the heat sink designs. First-principles characterization of metal foams as thermal management materials is undertaken. Heat and fluid flow characteristics of metal foams are examined through a macroscopic volume-averaged approach and also through a microscopic approach by modeling the foam geometry and interstitial fluid separately. In the macroscopic approach, small-scale details are ignored and the information so lost is represented in the governing equations using an engineering model. In this approach, two cases are considered: in the first, the fluid and metal foam are considered to be in thermodynamic equilibrium so that they may be described by a single temperature, and in the second, a two-temperature description is used to account for non-equilibrium behavior. The transient effects of different parameters governing the fluid and heat flow are investigated. In the microscopic approach, an improved representation of foam geometry is proposed and a detailed study of heat and fluid flow through the foam saturated by a fluid is presented. Detailed calculations of effective thermal conductivity, pressure drop and local Nusselt number are obtained. Further, the presence of pore-level gradients in temperature and velocity are often represented as an enhanced diffusion in macroscopic models. The enhanced diffusion contribution to the macro-scale transport is quantified through detailed pore-level heat and fluid flow simulations.

Degree

Ph.D.

Advisors

Murthy, Purdue University.

Subject Area

Mechanical engineering

COinS