Thermohydraulic performance of a periodic trapezoidal channel with a triangular cross-section

Simulations are performed to study the heat transfer behaviour of an equilateral triangular section duct following a tortuous path for fully-developed laminar flows with Reynolds numbers below 200. The enhancement of heat transfer and the increase in pressure drop are compared with those for ducts of circular, semi-circular and square section following the same serpentine path. For this flow regime, the triangular duct is shown to be the optimum choice (best heat transfer augmentation compared with increased pressure drop) amongst those studied. The effects of changing the path shape, the apex angle for an isosceles triangular cross-section and rounding of a corner of the equilateral triangular duct are also considered.     

Analysis of Evaporation Heat Transfer of Thin Liquid Film in a Capillary of Equilateral Triangular Cross—Section

IntroductionThe study on evaPoration heat transfer in caPillmpbodies is imPOrtant for design and development of higIilyefficient heat transfer equipment, such as heat pipe. P.C.W and Y.K. Kao[i] stUdied the interline heattransfer coefficients of an evaPorating wetting film.L.SoloVyev and S.A. KovalevI2l discussed the mechAnsmabout evaPoration of a liquld film from a porous sdse.A. Schonberg and P.C. Wayner['l developed Shae's(l953)['l evaPoration heat transfer model that based ontem…     

Thermal resistances of air-filled cavities of non-rectangular cross-section

Steady-state convective heat transfers across shaped, non-rectangular, closed-ended cavities containing air at atmospheric pressure have been measured. The thermal resistances of these are compared with previously published data for rectangular geometry cavities.     

Influence of tube cross-section geometry on high-pressure hydrogen-flow-induced self-ignition

Self-ignition within a cylindrical tube that discharges high-pressure hydrogen results in flame formation. Because rectangular tubes are used to visualize fuel-flow dynamics, the influence of the tube cross-section on the self-ignition characteristics is investigated. As experimental investigation of the mechanisms underlying the self-ignition phenomenon in cylindrical tubes is difficult, three-dimensional numerical simulation is employed. Following the bursting of diaphragm by high-pressure hydrogen with a storage pressure of 9.0 MPa and a temperature of 300 K, initial self-ignition occurs at the center of the rectangular tube sidewall. This is because of the mixing of air and hydrogen induced by the bow-shock reflection-generated jet flow and resulting adiabatic shock compression-induced temperature increase. This temperature rise induces a secondary self-ignition at the tube corners. In the cylindrical tube, a solitary ring-shaped self-ignition occurs near the sidewall. The flame evolutions in rectangular and cylindrical tubes reveal similar flame-spreading trends, which indicates similar bow-shock reflection-induced self-ignition mechanisms.     

On the deformation of the rectangular turbulent jet cross-section

The paper explains the reason for the experimentally observable deformation of a rectangular turbulent jet manifesting itself in a rapid growth (along the jet length) of the jet cross-section short side and a reduction of its long side. As a result, these sides change places some distance downstream of the jet origin. The jet deformation is shown to be due to a specific pressure field induced by large vortices that originate in the jet mixing zone.

A method of calculating the jet deformation is developed which makes use of the author's information on the pressure field produced by large vortices. The predicted results are compared with the experimental data available from other sources. The theory suggested applies to jet flows in boiler furnaces, dryers, combustion chambers of jet engines and stationary gas-turbine plants of electric power stations, chemical reactors, etc.     

Viscous flow in variable cross-section microchannels of arbitrary shapes

This paper outlines a novel approximate model for determining the pressure drop of laminar, single-phase flow in slowly-varying microchannels of arbitrary cross-section based on the solution of a channel of elliptical cross-section. A new nondimensional parameter is introduced as a criterion to identify the significance of frictional and inertial effects. This criterion is a function of the Reynolds number and geometrical parameters of the cross-section; i.e., perimeter, area, cross-sectional polar moment of inertia, and channel length. It is shown that for the general case of arbitrary cross-section, the cross-sectional perimeter is a more suitable length scale. An experimental investigation is conducted to verify the present model; 5 sets of rectangular microchannels with converging–diverging linear wall profiles are fabricated and tested. The collected pressure drop data are shown to be in good agreement with the proposed model. Furthermore, the presented model is compared with the numerical and experimental data available in the literature for a hyperbolic contraction with rectangular cross-section.     

三角形微槽中的气体滑移流动特性

针对三角形微槽利用正交函数法求解了带一阶滑移边界条件的N-S方程，对不可压燃气体在等腰三角形和等边三角形微槽道内的充分发展层流滑移流动特性进行了理论分析，获得了三角形微槽内的速度分布和阻力特性的分析解，计算结果表明，正交函数法适用于三角形微槽内滑移流动特性的分析计算，在滑移流区，三角形微槽边界上出现滑移流动，且随着Knudsen数（气体分子的平均自由程与流道特征尺寸之比）的增大，壁面上的滑移速度越大，流动阻力随之减小，但三角形微槽的三个角区边界上的滑移速度增加较小，三角形微槽的高宽比对无量纲阻力常数随Kn的变化关系影响很小。     

Visualization study of steam condensation in triangular microchannels

A visualization experiment is conducted to investigate the condensation of steam in a series of triangular silicon microchannels. The results indicate that droplet, annular, injection and slug-bubbly flow are the dominant flow patterns in these triangular silicon microchannels. With increased mass flow rate, or an increase in the hydraulic diameter under the same Reynolds number, the location at which the injection occurred is observed to move towards the channel outlet. The frequency of the injection increases, i.e. the flow of condensation instability is higher with increased inlet vapor Reynolds number, condensate Weber number and the prolongation of the injection location, or with a decrease in the hydraulic diameter of the channel. In addition, the wall temperature of the channel decreases along the condensation stream. The total pressure drop, the average condensation heat transfer coefficient and the average Nusselt number are observed to be larger with increased inlet vapor Reynolds number. Moreover, it is found that the condensation heat transfer is enhanced by a reduction in the channel scale.     

The analysis of triangular cycles of cooling and heating

Theoretical study of triangular cycle with cooling and heating at variable temperatures is investigated in this study. Simple approximated correlations for COP are obtained. By implementation of the cooling and heating using multistage machines, single-compressor machines accompanying with refrigerants mixtures, semiconductor thermopiles, and the supercritical cycles with variable temperature of heat rejection, the “triangular” cycle shows promising interesting features.     

Double-diffusive natural convection in a triangular solar collector

The current study deals with mathematical study for natural convection flow resulting from the combined buoyancy effects of thermal and mass diffusion inside a triangular shaped solar collector. The solution is performed assuming the isothermal and isoconcentration boundary conditions of absorbers and covers of collector. The two-dimensional governing equations for the physical phenomenon are expressed in a velocity–pressure formulation, along with the energy and concentration balance equations. A finite element method is used to solve these equations. Effects of the thermal Rayleigh number and buoyancy ratio are presented by streamlines, isotherms, isoconcentration as well as local and mean heat and mass transfer rates for the aforesaid parameters. Comparison with the previously published work is made and found to be an excellent agreement. Particularly, the design for enhancing the performance of the collector is determined by examining the abovementioned results.     

Thermal insulation provided by triangular sectioned attic spaces

The pitch of a triangular roof, apex upwards, should be about 15° in order to achieve a relatively high ratio of thermal insulation (in the absence of wind penetration) to capital investment. The thermal insulation provided by the contained air may be enhanced by incorporating a light, inverted T structure (suspended from the roof apex) because this inhibits the convective currents, as well as providing some radiation shielding.     

Slip flow forced convection in a microchannel with semi-circular cross-section

This paper offers theoretical results for fully developed slip-flow forced convection through a microchannel of semicircular cross-section. Numerical results are also presented to study the developing region. Velocity slip and temperature jump boundary conditions are applied at the uniformly heated walls. The results from the two different sources are cross validated and those pertaining to the limiting case of no-slip flow are found to be in good agreement with those available in the literature.     

Slip flow forced convection in a microporous duct of rectangular cross-section

Applying a Fourier series approach, closed form solutions for fully developed velocity and temperature distribution in a porous-saturated microduct of rectangular cross-section are presented in the slip flow regime. The Brinkman flow model is applied. Invoking the temperature jump equation, the H1 thermal boundary condition is investigated. Expressions are presented for the local and average velocity and temperature profiles, the friction factor, and the slip coefficient in terms of the key parameters. The present results, which are applicable to microducts of rectangular cross-section filled with or without a porous medium, are found to be in complete agreement with those available in the literature for both no-slip and slip flow regime.     

Constructal design of vertical multiscale triangular fins in natural convection

Constructal design of vertical multiscale triangular fins in natural convection is investigated in this paper. The design consists of two parts. The first part is for single-scale triangular fins. The objective in the first design is to reach to the highest heat transfer density from the fins for three fin angles (15°, 30°, and 45°). The single-scale fins are placed in a horizontal array and considered as isothermal fins. The degrees of freedom are the fin angle, and the fin-to-fin spacing. The constraint is the fin height. The second part is for multiscale fins where small fins are placed between the large fins which are optimized in the first part. In the second part, the angles of the large and small scales fins are kept constant at (15°). The optimal fin-to-fin spacing which is obtained in the first part is considered a constraint in the second part. The Rayleigh numbers in this design are (Ra = 10³, 10⁴, and 10⁵). The two-dimensional mass, momentum, and energy equations for natural convection are solved with the finite volume method. The results show that there is a benefit of placing the small-scale fins where the percentage increase in the heat transfer density is (10.22%) at (Ra = 10³), and (50.6%) at (Ra = 10⁵) due to existence of the small fins between the large fins.     

Three-dimensional numerical simulation of the slip flow through triangular microchannels

Three-dimensional numerical analysis for fully developed incompressible fluid flow and heat transfer through triangular microchannels over the slip flow regime is simulated in this paper. In order to study the flow through the channel, the Navier–Stokes equations are solved in conjunction with slip/jump boundary conditions. The influences of Knudsen number (0.001 < Kn < 0.1), aspect ratio (0.2 < A < 4.5), and Reynolds number (1 < Re < 15) on the fluid flow and heat transfer characteristics are extensively investigated in the paper. The numerical results reveal that the rarefaction decreases the Poiseuille number, while its effect on the Nusselt number completely depends on the interaction between velocity slip and temperature jump. It is also found that the aspect ratio has an important role in the analysis, but the variation of Reynolds number is less remarkable.     

Heat transfer to impinging round jets with triangular tabs

Experiments were performed to characterize the heat transfer enhancement produced by adding arrays of triangular tabs to the exit of turbulent round impinging jets issuing from a long pipe. For small nozzle-to-plate distances the local heat transfer was increased more than 25% in a series of distinct regions surrounding the impingement region. The largest increase in the average Nusselt number occurred for a nozzle-to-plate distance of approximately 4 diameter. In this case, the average Nusselt number was increased by 20% for the impingement region but only approximately 10% for the region with a radius of 3 jet diameters. Measurements of the velocity field were performed in free jets with tab arrays to investigate how the tabs modify the development of the flow.     

Free convection in porous media filled right-angle triangular enclosures

Steady-state free convection heat transfer in a right-angle triangular enclosure, whose vertical wall insulated and inclined and bottom walls are differentially heated, is performed in this study. The governing equations are obtained using Darcy model. In this study, the governing equations were solved by finite difference method and solution of algebraic equations was made via Successive Under Relaxation method. The effect of aspect ratios ranging from 0.25 to 1.0 and Rayleigh numbers 50 ≤ Ra ≤ 1000 is investigated as governing parameters on heat transfer and flow field. It is observed that heat transfer is increased with the decreasing of aspect ratio and multiple cells are formed at high Rayleigh numbers.