Forced convection heat transfer with water and phase-change-material slurries: Turbulent flow in a circular tube.

摘要

The objective of the present study was to understand the fundamental mechanism of the enhancement of the convective heat transfer coefficient as well as the thermal capacity of a working fluid by using the latent heat of the solid-liquid phase change of particles.; Tests were performed with water flowing turbulently in a long heating test section (i.e., 627 diameters) with a uniform heat flux boundary condition. Three effects influence the local friction and heat transfer coefficients: (1) the developing effect, (2) the radial viscosity change effect, and (3) the axial viscosity change effect. A new analysis method to obtain fully developed friction factor and Nusselt number is proposed. The Blasius equation, f = 0.079 Re{dollar}sp{lcub}-0.25{rcub},{dollar} was found to yield a good prediction of the turbulent friction coefficient with a 2.1% error. It would make a better prediction if the proportionality constant 0.079 in the equation was replaced with a new constant: 0.081. The power factor, m, in a general Nusselt number correlation, Nu = C Re{dollar}sp{lcub}rm m{rcub}{dollar} Pr{dollar}sp{lcub}0.4{rcub},{dollar} was found to be 0.979, which was greater than the widely used value of 0.8 used in the Dittus-Boelter correlation.; Tests were also conducted with a phase-change-material (PCM) slurry that was a suspension of PCM particles in a carrier fluid, water. A new method to generate very fine PCM particles using an emulsifier was introduced. With such fine PCM particles, the flow loop did not clog. Local pressure drops and local heat transfer coefficients were measured along the test section. Significant decreases in pressure drop occurred where the PCM particles in the slurry melted. The local convective heat transfer coefficient was found to vary significantly (up to 200%) when the particles melted. This made it difficult to directly apply the LMTD method or the effective thermal capacity method, which had been used in previous researches, to the heat transfer analysis of the PCM slurry flow. A new three-region melting model is proposed, and an explanation of the physical mechanism of the convective heat transfer enhancement due to the PCM particles is provided.