Third-grade non-Newtonian fluid flow and heat transfer in two coaxial pipes with a variable radius ratio with magnetic field

Purpose The purpose of this paper is to investigate the hydromagnetic third-grade non-Newtonian fluid flow and heat transfer between two coaxial pipes with a variable radius ratio. Design/methodology/approach To solve the approximate nonlinear and linear problems with variable coefficients, a trial function was applied. Methods include collocation, least square and Galerkin that can be applied for obtaining these coefficients. Findings It is revealed that an increase of the non-Newtonian parameter, Hartmann number, and radius ratio leads to an augmentation of the absolute value of the dimensionless velocity, temperature, velocity gradient, and temperature gradient of about 10-60%. Further, the augmentation of Bi1 reduces the absolute value of the dimensionless temperature profile and dimensionless temperature gradient about three to four times; hence, the dimensionless heat transfer rate reduces. However, the growth of Bi2 has a contrary impact. Besides, the increase of Pr and Ec leads to an increase in the dimensionless temperature profile and dimensionless temperature gradient; therefore, the dimensionless heat transfer rate increases. Originality/value The convection heat transfer on the walls of the pipes is considered, and the nonlinear coupled momentum and energy equations are solved using the least squared method and collocation methods, respectively.

Natural Convection MHD Effect on Heat Transfer in Vertical Cylinder

In this paper, natural convection MHD effect on Heat transfer in vertical cylinder is analyzed. Firstly heat transfer equation and convert it in dimensionless form by introducing dimensionless variables has considered. By use of finite Hankel and inverse Hankel transforms in dimensionless heat transfer equation, the values of temperature and Nusselt number is obtained. Finally, graphical representation is used to study the effect of Prandtl number pr on temperature and Nusselt number Nu. It is observed that the fluid at the center region of the cylinder is not heated for value of Prandtl number Pr greater than 1.7 for small values of t.

Investigation of laminar fluid flow and heat transfer of nanofluid in trapezoidal microchannel with different aspect ratios

Purpose In the present study, laminar steady flow of nanofluid through a trapezoidal channel is studied by using of finite volume method. The main aim of this paper is to study the effect of changes in geometric parameters, including internal and external dimensions on the behavior of heat transfer and fluid flow. For each parameter, an optimum ratio will be presented. Design/methodology/approach The results showed that in a channel cell, changing any geometric parameter may affect the temperature and flow field, even though the volume of the channel is kept constant. For a relatively small hydraulic diameter, microchannels with different angles have a similar dimensionless heat flux, while channels with bigger dimensions show various values of dimensionless heat flux. By increasing the angles of trapezoidal microchannels, dimensionless heat flux per unit of volume increases. As a result, the maximum and minimum heat transfer rate occurs in a trapezoidal microchannel with 75° and 30 internal’s, respectively. In the study of dimensionless heat flux rate with hydraulic diameter variations, an optimum hydraulic diameter (Dh) was observed in which the heat transfer rate per unit volume attains maximum value. Findings This optimum state is predicted to happen at a side angle of 75° and hydraulic diameter of 290 µm. In addition, in trapezoidal microchannel with higher aspect ratio, dimensionless heat flux rate is lower. Changing side angles of the channels and pressure drop have the same effect on pressure drop. For a constant pressure drop, if changing the side angles causes an increase in the rectangular area of the channel cross-section and the effect of the sides are not felt by the fluid, then the dimensionless heat flux will increase. By increasing the internal aspect ratio (t_2/t_3), the amount of t_3 decreases, and consequently, the conduction resistance of the hot surface decreases. Originality/value The effects of geometry of the microchannel, including internal and external dimensions on the behavior of heat transfer and fluid flow for pressure ranges between 2 and 8 kPa.

Thermal atomisation of a liquid drop after impact onto a hot substrate

This experimental study is focused on the mechanisms of thermal atomisation of a drop impacting onto a hot substrate. This phenomenon is characterised by the wetting and dewetting of the substrate, caused not by the rim dynamics, but induced by thermal effects. These thermal effects lead to the lamella evaporation, levitation and disintegration, generation of a vertical spray of fine droplets and consequently, drop breakup. A typical contact time of the drop before complete detachment is theoretically estimated. This estimation agrees very well with the experiments. It is shown that the Weber number, often used for describing splashing drops, is not a relevant parameter for thermal atomisation. Finally, a regime map is plotted, using a combination of the dimensionless contact time and the dimensionless heat flux at the substrate.

Constructal Design of Convective Y-Shaped Cavities by Means of Genetic Algorithm

In the present work constructal design is employed to optimize the geometry of a convective, Y-shaped cavity that intrudes into a solid conducting wall. The main purpose is to investigate the influence of the dimensionless heat transfer parameter a over the optimal geometries of the cavity, i.e., the ones that minimize the maximum excess of temperature (or reduce the thermal resistance of the solid domain). The search for the best geometry has been performed with the help of a genetic algorithm (GA). For square solids (H/L = 1.0) the results obtained with an exhaustive search (which is based on solution of all possible geometries) were adopted to validate the GA method, while for H/L ≠ 1.0 GA is used to find the best geometry for all degrees of freedom investigated here: H/L, t1/t0, L1/L0, and α (four times optimized). The results demonstrate that there is no universal optimal shape that minimizes the thermal field for all values of a investigated. Moreover, the temperature distribution along the solid domain becomes more homogeneous with an increase of a, until a limit where the configuration of “optimal distribution of imperfections” is achieved and the shape tends to remain fixed. Finally, it has been highlighted that the GA method proved to be very effective in the search for the best shapes with the number of required simulations much lower (8 times for the most difficult situation) than that necessary for exhaustive search.

The Study of Compression Engine Cycle Behavior in Terms of Variable Gazodynamic and Geometrical Configuration. Regime of the Maximum Economy

In this analysis, the variable configuration requires a variable distribution. This takes account of the areas offered by the valves and resistance gazodynamic coefficients. All at once the law of efficiency variation describes the variation of the power law. Cycle behavior was analyzed for the following values of the pressure drop coefficient on admission: 5%, 10%, 20% and 40%. For the calculations is necessary to determine the dimensionless heat developed by combustion. I used the following values: for the pressure drop coefficient on admission I use ψa=10 % and for the share of gazodynamic on the suction route conductance in total conductance I use .With this configuration imposed and with (dimensionless heat developed by combustion) resulted: (mean indicated pressure cycle) ; ηv = 0,876 (cylinder filling coefficient) ; α = 1,515 (excess air coefficient) and ηi = 0,499 (indicated cycle efficiency) ; (dimensionless indicated power ); (dimensionless heat taken combustion gas) ; (dimensionless heat taken environment). After numerical analysis result a maximum efficiency with relatively low dissipation.

Study of entropy generation in a slab with non-uniform internal heat generation

Analysis of entropy generation in a rectangular slab with a nonuniform internal heat generation is presented. Dimensionless local and total entropy generation during steady state heat conduction through the slab are obtained. Two different boundary conditions have been considered in the analysis, the first with asymmetric convection and the second with constant slab surface temperature. Temperature distribution within the slab is obtained analytically. The study investigates the effect of some relevant dimensionless heat transfer parameters on entropy generation. The results show that there exists a minimum local entropy generation but there does not exist a minimum total entropy generation for certain combinations of the heat transfer parameters. The results of calculations are presented graphically.