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Two Phase Flow Model And Conjugate Heat Transfer For Heat Pipe Heat Exchanger

Posted on:2009-08-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:D Z YuanFull Text:PDF
GTID:1101360242984596Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
Heat pipe demonstrates broad industrial applications for its high heat transfer efficiency. The fluid flow and heat transfer inside a heat pipe is a complex two-phase flow and phase change heat transfer process whose mechanism is not clear so far. Classical studies mostly employ experimental methods to investigate such a process while pure theoretical analysis is less reported. The theoretical investigation of an open thermosyphon based on fluid dynamics and heat transfer has just emerged in recent years.Conventional design and theoretical analysis for heat pipe heat exchanger (HPHE) just focus on the macroscopic balance calculation by using some empirical correlations or assuming the heat transfer characteristics fixed. However, the fluid flow and heat transfer properties of HPHE alter with the variation of the operating conditions and heat pipe arrangement. Therefore, the following four aspects of research work have been done in the present thesis.1. An open thermosyphon model is established by introducing and modifying the Mixture model. The effect of buoyancy is evaluated with the variable properties; the bubble diameter and departure diameter are calculated with empirical correlations and the interfacial area concentration in phase interfacial mass and energy source term is derived. Boiling heat transfer coefficient for an open thermosyphon computed by the present model agrees very well with the result from literature. Accordingly, pool boiling heat transfer coefficient for a single close thermosyphon is presented in terms of the relationship between the open and close thermosyphon boiling heat transfer coefficients.2. The fluid flow and heat transfer model is established for a single two-phase closed thermosyphon. In this model, the filmwise condensation and evaporation processes are simulated by modifying the Nusselt model involving the effects of interfacial shear force and surface subcooling degree, while the pool boiling process is simulated using the close thermosyphon pool boiling heat transfer model. A correlation for the overall thermosyphon heat transfer characteristics at low Re is obtained by compiling the three models with the mass and energy conservation. The solution result by the present model is more fit to the empirical one from literature compared to the Nusselt theoretical value. With this model, the effects of the geometrical structure, filling ratio, and properties of working liquid to the thermosyphon heat transfer characteristics are investigated and the inherent heat transfer mechanism of a single close thermosyphon is analyzed.3. Due to the interacting of the thermosyphon heat transfer characteristics and the external fluid temperature distribution, a 'coupled-source' heat transfer model is established. Heat pipes in the model are regarded as 'sourced' solid fields. The coupled boundary conditions of continous temperature and heat flux with that of external fluid are assumed which satisfies the practical situation. Hence, the heat transfer of either the cooling or the heating section can be viewed as a coupled convective heat transfer process between 'sourced' solid and the external fluid. The 'coupled-source' model provides reliable reference for the HPHE design and optimization.4. Since the fluid flow and heat transfer process of both cooling section and heating section have the same heat transfer principles, the present thesis takes the cooling section as an example to investigate the correlative factors affecting the HPHE heat transfer characteristics. The field synergy principle is also used for analyzing the effects of HPHE entrance conditions, the heat pipe geometrical arrangement and the operation temperature on the HPHE heat transfer characteristics. Combining the above two aspects of results, it shows that the fluid entrance conditions and the pipe geometrical arrangement are the key factors to the HPHE heat transfer performance and the empirically selected fluid entrance velocity is also verified theoretically.
Keywords/Search Tags:Coupled source model, Mixture model for two-phase flow, Phase change heat transfer, Heat pipe heat exchanger, Heat transfer enhancement
PDF Full Text Request
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