Revised K-ε Turbulent Model And Its Application For Turbulent Natural Convection In Enclosures

Acted by field force like gravity force, natural convection is induced by the buoyancy force resulting from temperature difference or concentration difference. It is a fluid flow and heat transfer phenomenon commonly occurring in many cases.Natural convection in enclosures has been received considerable atteinstruments, solar energy collectors design and nuclear reactor design. Significant numbers of experimental and numerical works had been carried out in the past decades with an attempt to deeply understand the turbulent flow and heat transfer in enclosures. However, the discrepancies between the numerical results and the experimental results of the turbulent natural convection in enclosure are still significant. With developing of computer power and numerical method, the numerical analysis, as a study method, becomes more and more indispensable. So, it is worth making more efforts to study turbulent model for natural convection in enclosures.A great numbers of the published investigations on turbulent natural convection in enclosures indicate that the discrepancy between the numerical results and the experimental results is significant using both the high-Reynolds-number k-ε model and the low-Reynolds-number k-ε model. Based on this situation, a series of works have been done in this dissertation:1. To find the reason that the discrepancy exists between the numerical results and the experimental results, the differences of forced convection and natural convection are analyzed from the aspect of the analogy between momentum transfer and heat transfer. According to the analysis, it is found that the definition of, at=vt/σt, should not be used to enclose the turbulent heat flux,-ρcpuT'in energy equation because the analogy between flow boundary layer and thermal boundary layer does not exist in natural convection.2. A composited k-ε model to resolve the turbulent natural convection heat transfer in enclosures is proposed. The composited k-ε model is used to numerically analyze the characteristics of fluid flow and heat transfer of natural convection in an air-filled enclosure. The numerical results are compared with the published experimental results and the numerical results, respectively. The results indicate that the composited k-ε model has a higher prediction accuracy than other models in some aspects, but the composited k-ε model should be further improved.3. To improve the accuracy of the composited k-ε model, turbulent Prandtl number is modified into σt=1+Pr, and then a revised turbulent model is formed. The revised turbulent model is used to analyze the characteristics of turbulent natural convection in enclosure and the results are compared with the published experimental results and numerical results, respectively. The comparisons indicate that the revised k-ε model can accurately predict the heat transfer characteristics of the hot and cold wall under the condition of Ra>109. Especially, when109<Ra≤1014, the averaged relative error between the numerical results and the published experimental results is equal to8.43%, which is much lower than any numerical results reported. In addition, on the condition of108≤Ra≤109, the numerical results of the revised k-ε model are in good agreement with the results reported in the published literatures.4. The revised k-ε model is used to numerically analyze the heat transfer characteristics of a heat exchanger used in a typical room for heating in Lanzhou region. More details of the effects of outerwall types, outerdoor environment conditions, heat exchanger surface area, heat transfer between neighbour rooms on the heat transfer characteristics of the heat exchanger, the surface temperature of the heat exchanger and the heat quantity supplied by the hat exchanger, the flow field and temperature field of the room arc obtained.