The Performance Study On Heat Transfer Enhancement Of Microencapsulated Phase Change Material Suspension With Laminar Flow In A Circular Tube

Microencapsulated phase change material suspension is a kind of functionallylatent thermal fluid, which consists of the microencapsulated particles of phasechange material(PCM)and the carrier fluid and serves as both energy storage mediaand heat transfer media, it has important applications in temperature control, energystorage and energy saving systems. In this thesis, based on a comprehensive surveyfor both domestic and overseas studies available in this field, the mechanism of heattransfer enhancement of microencapsulated phase change material suspension withlaminar flow in a circular tube is analyzed theoretically and numerically. Then, theinfluence factors governing the ratio of heat transfer enhancement are analyzed andthe quantitative effects are calculated numerically. Moreover, the basic characteristicsof phase change process occurring on a single microcapsule are investigated.Based on the comprehensive review for previous researches concerning the areaof microcapsule heat transfer, the contents and the methods used in this thesis areconfirmed. "Field synergy principle", that aims at heat transfer enhancement of singlephase fluid, is used to analyze and explain the convective heat transfer enhancementmechanism of microencapsulated phase change material suspensions in a circular tubein the following two aspects. Firstly, if microencapsulated phase change materialsuspensions is regarded as a homogeneous fluid, the phase change latent heat of PCMis equivalent to a huge apparent specific heat within the given temperature range,convective heat transfer enhancement is occurred. Secondly, "micro-convectioneffect" between the encapsulated particles and surrounding carrier fluid promotessignificant enhancement of convective heat transfer between the particles and thecarrier fluid, in fact, this type of enhancement can be regarded as the increase of theeffective conductivity of the suspensions.An equivalent specific heat model of microencapsulated phase changesuspensions with laminar flow in a circular tube having constant heat flux andconstant wall temperature is developed. According to the result of numericalcalculation, all factors affecting the degree of heat transfer enhancement are discussedin detail. It is discovered that Stefan number Ste and the volumetric concentration ofmicrocapsules C are two most important factors influencing the heat transferenhancement. Compared with the congener experiments or numerical simulations byother investigators, the computational solutions in this thesis are proved to be correctand credible.A mathematical model that reflects the phase change process of singlemicrocapsule is established. The numerical simulation of the phase change process iscarried out by both enthalpy method and fixed step length method. Throughexamining the microcapsule's shell thickness, the conductivity of the shell materialand the temperature difference of convective heat transfer between encapsulatedparticles and surrounding carrier fluid, the basic characteristics of solid-liquid phasechange process inside single microcapsule are fully understood. The simulated resultsindicate that influence of microcapsule's shell thickness and the conductivity of shellmaterial, within a certain range, on the phase change process is very limited.Lastly, some ideas and suggestions about the research direction of this field inthe future are put forward in this thesis.