Chakkrit Na Ranong

Consideration of maldistribution in heat exchangers using the hyperbolic dispersion model

Axial temperature profiles in a shell and tube heat exchanger are numerically calculated for given maldistributions on the tube side. For comparison the same maldistributions are handled with the parabolic and hyperbolic dispersion model with fitted values for the axial dispersion coefficient and third sound wave velocity. The analytical results clearly demonstrate that the hyperbolic model is better suited to describe the steady state axial temperature profiles.

Approach for the Determination of Heat Transfer Coefficients for Filling Processes of Pressure Vessels With Compressed Gaseous Media

For fast and effective simulation of filling processes of pressure vessels with compressed gaseous media, the governing equations are derived from a mass balance equation for the gas and from energy balance equations for the gas and the wall of the vessel. The gas is considered as a perfectly mixed phase and two heat transfer coefficients are introduced. The first one is the mean heat transfer coefficient between the gas and the inner surface of the pressure vessel, and the second one is the heat transfer coefficient between outer surface of the vessel and the surroundings. Because of the heat capacity of the wall of the pressure vessel, heat transfer from the compressed gas to the vessel wall strongly influences the temperature field of the gas. Until now no correlations have been available for the heat transfer coefficient between inflowing gas and inner surface of the vessel. To solve this problem, a computational fluid dynamics tool is used to determine the gas velocities at the vicinity of the inner surface of the vessel for a number of discrete surface elements. The results of a large amount of numerical experiments show that there exists a unique relationship between the gas velocity at the inlet and the tangential fluid velocities at the vicinity of the inner surface of the vessel for each vessel geometry. Once this unique relationship is known, the complete velocity distribution at the vicinity of the inner surface can be easily calculated from the inlet gas velocity. The near-wall velocities at the outer limit of the boundary layer are substituted into the heat transfer correlation for external flow over flat plates. The final heat transfer coefficient is the area-weighted mean of all local heat transfer coefficients. The method is applied to the special case of filling with hydrogen a 70-MPa composite vessel for fuel cell vehicles.

Steady-state and transient behaviour of two heat exchangers coupled by a circulating flowstream

Systems consisting of two heat exchangers coupled by a circulating flowstream are studied. The systems differ in the flow configurations of the single heat exchangers. For steady-state operation there exists a heat capacity rate of the circulating flowstream which maximizes the temperature changes of the external flowstreams. Until now this optimum has been calculated, assuming that the overall heat transfer coefficients of the heat exchangers do not depend on the mass flow rate of the circulating flowstream. In this paper the dependence of the overall heat transfer coefficient on mass flow rate of the circulating flowstream is taken into account. For transient operating conditions the system response to perturbations of inlet temperatures and mass flow rates is calculated by the method of Laplace-transforms and an explicit finite difference method. The most significant features of the coupled system become apparent considering outlet temperature transients induced by perturbations of the mass flow rate of the circulating flowstream.

Steady-State and Transient Behavior of Two Heat Exchangers Coupled by a Circulating Flowstream

Systems consisting of two heat exchangers coupled by a circulating flowstream are studied. The systems differ in the flow configurations of the single heat exchangers. For steady-state operation, there exists a heat capacity rate of the circulating flowstream that maximizes the temperature changes of the external flowstreams. Until now, this optimum has been calculated assuming that the overall heat transfer coefficients of the heat exchangers do not depend on the mass flow rate of the circulating flowstream. In this paper, the dependence of the overall heat transfer coefficient on the mass flow rate of the circulating flowstream is taken into account. For transient operating conditions, the system response to perturbations of inlet temperatures and mass flow rates is calculated by the method of Laplace Transforms and an explicit finite difference method. The most significant features of the coupled system become apparent by considering outlet temperature transients induced by perturbations of the mass flow rate of the circulating flowstream.