In this article, a numerical investigation of the flow and heat transfer in a three-row finnedtube heat exchanger is conducted with a three-dimensional laminar conjugated model Four types of fin surfaces are studied; one is the whole plain plate fin, and the other three are of slotted type, called slit 1, slit 2, and slit 3. AU four fin surfaces have the same global geometry dimensions. The three slotted fin surfaces have the same numbers of strips, which protrude upward and downward alternatively and are positioned along the flow direction according to the rule of "front coarse and rear dense." The difference in the three slotted fins is in the degree of "coarse" and "dense" along the flow direction. Numerical results show that, compared to the plain plate fin, the three types of slotted fin all have very good heat transfer performance in that the percentage increase in heat transfer is higher than that in the friction factor. Among the three siotted fin surfaces, slit 1 behaves the best, followed by slit 2 and slit 3 in order. Within the Reynolds number range compared (from 2,100 to 13,500), the Nusselt number of slit 1 is about 112-48% higher than that of the plain plate fin surface under the identical pumping constraint. An analysis of the essence of heat transfer enhancement is conducted from the field synergy principle, which says that the reduction of the intersection angle between the velocity and the temperature gradient is the basic mechanism for enhancing convective heat transfer. It is found that for the three comparison constraints the domain-average synergy angle of slit 1 is always the smallest, while that of the plain plate fin is the largest, with slit 2 and slit 3 being somewhat in between. The results of the present study once again show the feasibility of the field synergy principle and are helpful to the development of new types of enhanced heat transfer surfaces.

Abstract Based on the normalized variable diagram, the weakness of the Gaskell and Lau's convective boundedness criterion (GL-CBC) is revealed by numerical example. By careful consideration of the smoothness of the normalized variable variation pattern, more rigorous constraints on the interface value interpolation are found. A new CBC is thus proposed, whose feasibility and correctness are demonstrated by the inspection of ten existing bounded schemes and a numerical example.

The steady natural-convection heat transfer in a tilted cylindrical envelope with constant but different end temperatures (300 and 80K) is investigated numerically with the lateral surface being adiabatic. The inner diameter of the envelope is 27.8mm and its length/diameter ratio is 9. This is supposed to be a simplified model for the pulse tube in a pulse-tube cryocooler when the pulse tube is positioned at different orientations. The problem studied is a typical nonlinear one in that the thermophysical properties of the working fluid (helium) vary significant from the hot end to the cold end. Three-dimensional steadystate governing equations are solved with fully variable thermal properties. The high nonlinearity of the problem leads to many special characters of the convergence process, and a very peculiar convergence process is found. Initial-field dependence is also revealed. After quite a few preliminary computations, a series of convergence criteria are proposed. Grid-independence examination is conducted for inclination angle of 110°. It is found that the grid system of 20(r)×20(ψ)×80(Z) with grids in the z direction being nonuniformly positioned can obtain a grid-independent solution. Preliminary computations are conducted for the horizontal position with 70℃ of end temperature difference. The predicted velocity and temperature distributions are compared with available measured data. Good agreement between the predicted and measured results provides strong support for the physical model and numerical treatments developed in this article.

Existing methods for analyzing the stability of a discretized scheme for convection.diffusion terms are usually based on five assumptions, i.e., one-dimensional, linear, first kind of boundary condition, source term free, and uniform grid system. In this article we examine numerically whether deviation from one of the assumptions may enhance the stability of the discretized scheme. The second part of the article is devoted to the criterion of convective boundedness. It is shown that the convective boundedness criterion (CBC) proposed by Gaskell and Lau is only a sufficient condition. Another region in the normalized variable diagram is proposed within which any scheme defined is convectively bounded. Three new bounded high-resolution schemes defined in this region, SBECBC1, 2, and 3, are proposed, and numerical experiments for two advection problems and one diffusion-convection problem demonstrate the high, resolution ability of the SBECBCs for a sharp change in scalar profile.

A new method is proposed to accelerate the convergence rate for the SIMPLER algorithm by arti