The effect of varying chill surface roughness on interfacial heat transfer during casting solidification

Experiments to investigate interfacial heat transfer mechanisms during casting solidification were carried out by varying the surface roughness of a Cu chill used to bring about unidirectional solidification of an Al-4.5 wt.% Cu alloy. Little variation in interfacial heat transfer coefficient with varying chill surface roughness was found, confirming previously published results. Examination of the as-cast surface of the casting showed the presence of predendritic contact areas, and also that the casting surface roughness did not form as a replica of the chill surface, as has often been proposed. Rather, the casting surface roughness was consistently greater than that of the chill, but varied little in the experiments. A sum surface roughness parameter was devised to characterise the casting–chill interface that included the roughness of both surfaces. The value of this parameter was strongly influenced by the greater roughness of the casting surface, rather than the chill surface roughness, and therefore also varied little in the experiments. This lack of variation in the casting surface roughness and hence the sum surface roughness parameter showed how interfacial heat transfer should be more strongly influenced by the greater roughness of the casting surface than of the chill surface, and explains why the interfacial heat transfer coefficient was not strongly influenced by the chill surface roughness in these types of experiments. At the time the work was carried out the authors were in the Manchester Materials Science Centre, University of Manchester and UMIST, Manchester M1 7HS, UK.     

Numerical-analytical modeling of heat transfer between a laminar flow and highly permeable roughness

A mathematical model is proposed for the thermal interaction of a laminar flow with a layer of small stationary streamlined obstacles. Numerical and analytical investigations exhibit characteristic zones of the flow.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 5, pp. 791–797, November, 1985.     

Hydromagnetic flow and heat transfer on a continuously moving vertical surface

Summary A theoretical solution for hydromagnetic convection over a continuously moving vertical surface with uniform suction is obtained. A flow of this types represents a new class of boundary-layer flow at a surface of finite length. The solutions for the velocity and temperature profiles are obtained. It is observed that the velocity decreases considerably in the presence of a magnetic field, as compared to its absence.     

Single-layer model for surface roughness

Random roughness of an optical surface reduces its specular reflectance and transmittance by the scattering of light. The reduction in reflectance can be modeled by a homogeneous layer on the surface if the refractive index of the layer is intermediate to the indices of the media on either side of the surface. Such a layer predicts an increase in the transmittance of the surface and therefore does not provide a valid model for the effects of scatter on the transmittance. Adding a small amount of absorption to the layer provides a model that predicts a reduction in both reflectance and transmittance. The absorbing layer model agrees with the predictions of a scalar scattering theory for a layer with a thickness that is twice the rms roughness of the surface. The extinction coefficient k for the layer is proportional to the thickness of the layer.     

The effect of heat transfer additive and surface roughness of micro-scale hatched tubes on absorption performance

The objectives of this paper are to investigate the effect of heat transfer additive and surface roughness of micro-scale hatched tubes on the absorption performance and to provide a guideline for the absorber design. Two different micro-scale hatched tubes and a bare tube are tested to quantify the effect of the surface roughness on the absorption performance. The roughness of the micro-scale hatched tubes ranges 0.39–6.97 μm. The working fluid is H₂O/LiBr solution with inlet concentration of 55, 58 and 61 wt.% of LiBr. Normal Octanol is used as the heat transfer additive with the concentration of 400 ppm. The absorber heat exchanger consists of 24 horizontal tubes in a column, liquid distributor at the liquid inlet and the liquid reservoir at the bottom of the absorber. The effect of heat transfer additive on the heat transfer rate is found to be more significant in the bare tube than that in the micro-scale hatched tubes. It is found that the absorption performance for the micro-hatched tube with heat transfer additive becomes up to 4.5 times higher than that for the bare tube without heat transfer additive. It is concluded that the heat transfer enhancement by the heat transfer additive is more significant than that by the micro-scale surface treatment.     

Pool boiling heat transfer at finned tubes: influence of surface roughness and shape of the fins

Pool boiling heat transfer from finned tubes with different shapes of fins (trapezoid-shaped, T-shaped, or Y-shaped) to various hydrocarbons and partly fluorinated hydrocarbons has been investigated at the Laboratorium für Wärme- und Kältetechnik, Universität-GH Paderborn during the recent past. Compared to corresponding measurements on plain tubes, heat transfer on traditionally finned tubes with trapezoid-shaped fins is considerably improved, and even better results are achieved with T-shaped or Y-shaped fins. The influences of the macrostructure (i.e. fin geometry) or microstructure (i.e. surface roughness) on the heat transfer coefficient have been studied separately, in order to evaluate the improvement of heat transfer by either influence.     

An approximate mathematical model of heat and mass transfer in two-phase flow

We propose an approximate method for studying transport processes in one-dimensional two-phase flow which permits the determination of the system output as a function of input and the system parameters. The error of the method is estimated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 4, pp. 673–683, April, 1978.     

Combined heat and mass transfer in natural convection flow from a vertical wavy surface

Summary In the present paper, effects of combined buoyancy forces from mass and thermal diffusion by natural convection flow from a vertical wavy surface have been investigated using the implicit finite difference method. Here we have focused our attention on the evolution of the surface shear stress,f(0), rate of heat transfer,g(0), and surface concentration gradient,h(0) with effect of different values of the governing parameters, such as the Schmidt number Sc ranging from 7 to 1500 which are appropriate for different species concentration in water (Pr=7.0), the amplitude of the waviness of the surface ranging from 0.0 to 0.4 and the buoyancy parameter,w, ranging from 0.0 to 1.Notation C species concentration in the boundary layer - C species concentration of the ambient fluid - C _w species concentration at the surface - D chemical molecular diffusivity - f dimensionless stream function - g acceleration due to gravity - Gr _x local modified Grashof number - N ratio of the buoyancy forces due to the temperature difference and the concentration difference - p pressure of the fluid - T temperature of the fluid in the boundary layer - T temperature of the ambient fluid - T _w temperature at the surface - u, v thex- andy-components of the velocity field - x, y axis in the direction along and normal to the plate Greek symbols thermal diffusivity - _T volumetric coefficient of thermal expansion - _C volumetric coefficient of expansion with concentration - stream function - nondimensional similarity variable - x/L - density of the ambient fluid - v kinematic coefficient of viscosity - stream function - dimensionless skin friction - fluid viscosity     

Quantifying the surface roughness effect in microindentation using a proportional specimen resistance model

The influence of the surface roughness on the indentation size effect in microindentation was examined using the proportional specimen resistance model. Stainless steel, aluminium, and copper surfaces were polished to different levels of roughness and subjected to microindentation. The results showed that the indentation size effect increases with increasing surface roughness, according to the proportional specimen resistance model. A normalized hardness equation H/H ₀ = (c ₀ + c ₁ R _a)/(a ₂ d) + 1 was established, and the value of c ₁ can be used to quantify the effect of surface roughness on the severity of the indentation size effect; this value was found to be highest for stainless steel, followed by copper and aluminium.     

Numerical model of a parallel flow minichannel evaporator with new flow boiling heat transfer correlation

In this paper, a distributed parameter (DP) numerical model with the new proposed flow boiling heat transfer correlation was established for parallel flow minichannel (PFMC) evaporator. DP model validation was made by comparing the measured values obtained on experimental studies, which were conducted under refrigerant mass flow rate range of 34.6–245.6 kg h⁻¹ and evaporation pressure of 200–500 kPa. The effects of four different flow boiling heat transfer correlations on DP model performance were investigated. Results showed that the new correlation predicted 99% of experimental data in ± 30% error bands. Moreover, the DP model with the new correlation yielded the mean absolute error (MAE) of 1.5%, 9.1%, 18.8%, 14.2% and 19.8% in prediction of cooling capacity, outlet air temperature, refrigerant superheat, air side and refrigerant side pressure drop, respectively. The presented DP model can be implemented to evaluate the performance of PFMC evaporator, and therefore can save efforts on component and system design and optimization.     

Modelling heat transfer in two‐fluid interfacial flows

This paper presents a method to calculate heat transfer across an interface separating immiscible fluids. A volume tracking method was used to model the simultaneous movement of mass, momentum and energy across cell boundaries. Both first‐ and second‐order methods were used to approximate temperature fields with sharp gradients that exist near the fluid–fluid interface. Temperature distributions around hot droplets surrounded by a colder fluid with uniform velocity were calculated and the magnitude of false diffusion identified. The effect of changing the thermal diffusivity of the surrounding fluid was studied. It was found that in most cases a second‐order approximation, such as the van Leer scheme, is sufficient for estimating advection temperatures. To demonstrate the capabilities of the model we modelled molten tin droplets falling in an oil bath. The development of vortices behind droplets was modelled and the effect of fluid re‐circulation and oil thermal conductivity on heat dissipation studied. Copyright © 2004 John Wiley & Sons, Ltd.     

Non-Darcian flow and heat transfer along a permeable vertical surface with nonlinear density temperature variation

The non-Darcy free convection flow on a vertical flat plate embedded in a fluid-saturated porous medium in the presence of the lateral mass flux with prescribed constant surface temperature is considered. The coupled nonlinearities generated by the density variation with temperature, inertia, and viscous dissipation are included in the present study. In particular, we analyze a system of nonlinear ODEs describing self-similar solutions to the flow and heat transfer problem. These transformed equations are integrated numerically by a second-order finite difference scheme known as the Keller box method. Furthermore, some analytical results are provided to establish relationships between the physical invariants in the problem, and also to validate the numerical method. One of the important findings of our study is that an increase in the Rayleigh number increases the velocity boundary layer thickness, while the opposite is true for the thermal boundary layer thickness.     

Hydromagnetic flow and heat transfer over a continuous,moving, porous,flat surface

Summary Exact solutions for hydromagnetic boundary-layer flow and heat transfer over a continuous, moving, flat surface with uniform suction and internal heat generation/absorption are obtained. Flow of this type represents a new class of boundary-layer problems, with solutions substantially different from those for boundary-layer flow on a flat surface of finite length. These solutions are even exact solutions of the complete Navier-Stokes equations and the energy equation.With 3 Figures     

A surface roughness model for a turning operation

A mathematical model for the surface roughness in a turning operation was developed in terms of the cutting speed, feed and depth of cut. Utilizing PL1 language and an IBM 360/50 computer, the model was used to generate contours of surface roughness in planes containing the cutting speed and feed at different levels of depth of cut. The surface roughness contours were used to select the machining conditions at which an increase in the rate of metal removal was achieved without sacrifice in surface finish.     

Numerical study on heat transfer and entropy generation of developing laminar nanofluid flow in helical tube using two-phase mixture model

Nanofluids and helical tubes are among the best methods for heat transfer enhancement. In the present study, laminar, developing nanofluid flow in helical tube at constant wall temperature is investigated. The numerical simulation of Al₂O₃-water nanofluid with temperature dependent properties is performed using the two-phase mixture model by control volume method in order to study convective heat transfer and entropy generation. The numerical results is compared with three test cases including nanofluid forced convection in straight tube, velocity profile in curved tube and Nusselt number in helical tubes that good agreement for all cases is observed. Heat transfer coefficient in developing region inside a straight tube using mixture model shows a better prediction compared to the homogenous model. The effect of Reynolds number and nanoparticle volume fraction on flow and temperature fields, local and overall heat transfer coefficient, local entropy generation due to viscous dissipation and heat transfer, and the Bejan number is discussed in detail and compared with the base fluid. The results show that the nanofluid and the base fluid have almost the same axial velocity profile, but their temperature profile has significant difference in developing and fully developed region. Entropy generation ratio by nanofluid to the base fluid in each axial location along the coil length showed that the entropy generation is reduced by using nanofluid in at most length of the helical tube. Also, better heat transfer enhancement and entropy generation reduction can be achieved at low Reynolds number.     

Finite element model for beef chilling using CFD-generated heat transfer coefficients

A combined model of the beef chilling process is presented, in which computational fluid dynamics (CFD) was used to estimate the local heat and mass transfer coefficients, assuming uniform surface temperatures, and a set of 2-D finite element grids was used to solve the heat transfer equation in the product, which has an elongated shape. Another set of 1-D grids was used to solve the water transport equation near the surface of the meat. The surface transfer coefficients were calculated for various combinations of air orientations and speeds, and summarised in a set of regression equations. The model was verified by existing and new data on heat load, temperatures, weight loss and surface water activity.