Search results

A numerical study of natural convection of air in a vertical annulus has been conducted, where the inner wall is heated with constant heat flux at its inner side, the outer wall of the annulus being maintained at constant temperature, and the top and bottom plates are assumed to be insulated. The cases of radius ratio K = 3, aspect ratio A = 10∼30, and Ra* = 103∼1.7 × 107 have been simulated. Both axial conduction and surface radiation are taken into account to reveal their effects on the distributions of inner wall temperature and local Nusselt number. Emphasis is on the comparison between the numerical results and the relevant experimental data, and the comparison between numerical solutions with and without considering the surface radiation. The numerical results of heat transfer are found to be in good agreement with the corresponding experimental results in the literature. The dependence of average relative conductivity on aspect ratio and the effect of imperfection in top and bottom insulation on the inner wall temperature are also discussed.

The effect of the tube wall heat conduction on the natural convection in a tilted long cylindrical envelope with constant, but different temperature of the two ends and an adiabatic outer surface was numerically investigated. The envelope is supposed to be a simplified model for the pulse tube in a pulse tube refrigerator when the pulse tube is positioned at different orientations. It is found that the cylindrical envelope lateral wall heat conduction can enhance the heat transfer from the hot end to the cold end, not only because of the increase in pure heat conduction in the wall, but more importantly, also the intensification of the natural convection within the enclosure. This enhancement is resulted from the big temperature difference between the tube wall and the adjacent fluid near the hot and cold ends. Adoption of low thermal conductivity tube can effectively reduce such additional heat transfers from hot to cold end, thus reducing the loss of cooling capacity for the pulse tube refrigerator.

To provide an improved version of SIMPLER algorithm which can enhance the convergence rate of the iterative solution procedure in the field of computational fluid dynamics analysis.

The improved version of SIMPLER algorithm is developed by modifying the coefficients of the velocity correction equation and implementing the correction of pressure within an iteration cycle.

The CSIMPLER algorithm (the improved version) can enhance the convergence rate for almost all cases tested, especially for the low under‐relaxation factor situations. The pressure correction term even can be overrelaxed to further enhance the convergence rate.

The CSIMPLER algorithm can enhance the rate of convergence to different degree for different problems. It can only be adopted to solve the incompressible fluid flow and heat transfer.

CSIMPLER is a simple and effectual method to enhance the convergence rate of the iterative process for the computational fluid dynamics analysis. The existing code of SIMPLER can be easily changed to CSIMPLER.

The paper developed an improved version of SIMPLER algorithm with some minor changes in the existing SIMPLER code.

For a thermal protection system (TPS) of long endurance hypersonic flight vehicle (HFV), its thermal insulation property not only determines by the manufactured morphology but also changes along time. A thermal conductivity prediction model for aerogel considering heat treatment effect is carried out and applied to solve the heat conduction problem of a TPS. The aim of this study is to provide theoretical and numerical references for further development of aerogels applying to TPSs.

A thermal conductivity prediction model for aerogel is established considering treatment effect. The heat conduction problem of a TPS is derived and solved by combining the differential quadrature method and the Runge–Kutta method. The prediction results of aerogel thermal conductivities are verified by comparing with those in literature, while the calculated temperature field of TPS is verified by comparing with that by ABAQUS.

Numerical results show that when applying the current prediction model, the calculated high temperature area in the aerogel layer is narrowed due to the decrease of the thermal conductivity during heat treatment process.

This study will be beneficial to carry out the precise design of TPS for long endurance HFVs.

The periodically fully developed laminar heat transfer and fluid flow of an array of non‐uniform plate length, aligned at angles to air flow direction, have been investigated by numerical analysis in the Reynolds number range from 15 to 410. To implement the periodic boundary condition in both streamwise and spanwise directions and to confine the computation exactly within one cycle, proposes a linear interpolation technique, which has been proved successful by comparing the results with those obtained in an extended domain. It has been found that with an increase in the ratio of the long plate length to that of the short plate and with a decrease in the ratio of the transverse pitch to the streamwise cycle length, both the Nusselt number and friction factor decrease. Comparison of the numerical results of Nusselt number and friction factor with relevant experimental results have been performed. The agreement should be judged reasonably good.

A comparative study is performed to reveal the convergence characteristics and the robustness of four variants in the semi‐implicit method for pressure‐linked equations (SIMPLE)‐family: SIMPLE, SIMPLE revised (SIMPLER), SIMPLE consistent (SIMPLEC), and SIMPLE extrapolation (SIMPLEX). The focus is concentrated in the solution at fine grid system. Four typical fluid flow and heat transfer problems are taken as the numerical examples (lid‐driven cavity flow, flow in an axisymmetric sudden expansion, flow in an annulus with inner surface rotating and the natural convection in a square enclosure). It is found that an appropriate convergence condition should include both mass conservation and momentum conservation requirements. For the four problems computed, the SIMPLEX always requires the largest computational time, the SIMPLER comes the next, and the computational time of SIMPLE and SIMPLEC are the least. As far as the robustness is concerned, the SIMPLE algorithm is the worst, the SIMPLER comes the next and the robustness of SIMPLEX and SIMPLEC are superior to the others. The SIMPLEC algorithm is then recommended, especially for the computation at a fine grid system. Brief discussion is provided to further reveal the reasons which may account for the difference of the four algorithms.

A numerical study has been conducted for the characteristics of the periodically fully developed turbulent flow and heat transfer in a channel with transverse opposite‐positioned fins. The Reynolds number range is 2 × 10⁴ to 7 × 10⁴. K‐ε model and wall function method were adopted during the calculation. The influence of the thermal boundary condition of the fin to the heat transfer has been verified. For the studied configuration the prominent feature that differs from the similar laminar heat transfer is the phenomenon of secondary peak of the Nusselt number distribution. Assessment of heat transfer enhancement under the constraint of the same pump power reveals that the effect of the configuration of the relative fin height, e/H, equal to 0.1 is superior to those of e/H equal to 0.15 and 0.2. Comparing with the results of the channel with rod disturbances, the studied configuration possesses nearly the same heat transfer enhancement effect. Transient simulations to cases with big fin have also been conducted to assure the validity of the steady algorithm.

A computational study of convective heat transfer for turbulent flows in multi‐channel, narrow‐gap fuel element has been carried out, using a general marching procedure. The fluid distribution adjustment among seven annular‐sector channels is based on the assumption of the same pressure drop in these passages. It was found that the inlet velocities of the bilateral channels are lower than those of the middles, and the axial local heat transfer coefficients for the seven channels do not approach the fully developed constant value. At each cross section, the periphery temperature distribution is not uniform, while the local temperature distribution along axial coordinate is of sinuous type with the peak at x=0.7–0.8 m. At the same Reynolds number, the averaged Nusselt numbers of water in Channel 1 and Channel 7 are higher than those in the middles. The maximum surface temperature increases almost linearly with the inlet water temperature, whereas it decreases almost asymptotically with the inlet average velocity.

To provide some heat transfer and friction factor results for fin‐and‐tube heat transfer surfaces which may be used in air conditioning industry.

Numerical simulation approach was adopted to compare the plain plate fin and three types of radial slotted fin surfaces.

It is found that at the same frontal velocity (1.0‐3 m/s) the plain plate fin has the lowest heat transfer rate with the smallest pressure drop. The full slotted fin surface has the highest heat transfer rate with the largest pressure drop penalty. The partially slotted fin (where the strips are mainly located in the rear part of the fin) and the back slotted fin are some what in between. Under the identical pumping power constraint, the partially slotted fin surface behaves the best.

The results are only valid the two‐row fin surface.

The results are very useful for the design of two‐row tube fin surfaces with high efficiency.

This paper provides original information of slotted fin surface with radial strips from the field synergy principle.

Heat transfer inside wavy fins is analyzed in this work. The paper aim to discuss this issue.

Six different types of wavy fins are considered. The fin equation for each fin type is solved using a high accurate finite difference method. Excellent agreement is obtained between the numerical solution under zero wave amplitude and the exact solution of the plain fin.

The following wavy fin types and conditions are found to produce larger heat transfer rate and its volumetric value than those for the plain fin and other wavy fins: short fins with parallel wavy profiles and large surface-wave frequency; long fins with symmetric wavy surface around the length axis, positive cross-sectional area gradient at the base, and large surface-wave frequency; and long fins with symmetric wavy profiles around the length axis, positive cross-sectional area gradient at the base, and small surface-wave frequency.

In addition, both fins with symmetric wavy surface around the width axis and parallel wavy surfaces along the width axis have same performance indicators. Also, these wavy fins possess higher fin efficiency than either that of the plain fin or those of the other types of wavy fins.

Finally, heat transfer enhancements in the studied wavy fins are increased by increases in the excess of the surface area, cross-sectional area gradient at the base, arc length and arc width relative to those of the plain fin.