S. Savino

EFFECT OF ASPECT RATIO ON CONVECTION ENHANCEMENT IN WAVY CHANNELS

Pressure drop and heat transfer characteristics are investigated in the fully developed region of three-dimensional wavy channels whose aspect ratios (width over height) range from one to infinity. Numerical simulations show that Nusselt numbers and friction factors increase with decreasing aspect ratios, in such a way that global performances improve. In all the channels considered, friction factors always increase with the Reynolds number, while Nusselt numbers significantly increase only above the critical value of the Reynolds number at which self-sustained flow oscillations begin. In turn, the critical value of the Reynolds number decreases with the aspect ratio, down to a minimum that is reached asymptotically.

Convective heat and mass transfer in wavy finned‐tube exchangers

The paper adopts a simplified two‐dimensional approach to deal with convective heat and mass transfer in laminar flows of humid air through wavy finned‐tube exchangers. The computational domain is spatially periodic, with fully developed conditions prevailing at a certain distance from the inlet section. Both the entrance and the fully developed flow region are investigated. In the fully developed region, periodicities in the flow, temperature and mass concentration fields are taken into account. The approach is completely general, even if the finite element method is used for the discretizations. In the application section, velocity, temperature, and mass concentration fields are computed first. Then apparent friction factors, Nusselt numbers, Colburn factors for heat and mass transfer, and goodness factors are evaluated both in the entrance and in the fully developed region.

Effect of space ratio and corrugation angle on convection enhancement in wavy channels

By neglecting the influence of tubes, this paper adopts a simplified two‐dimensional approach to deal with laminar convection of air through wavy finned‐tube exchangers. Pressure drop and heat transfer characteristics are investigated in the fully developed region of the flow channels between adjacent fins. The solutions are presented for several space ratios (height over length of a module) and two corrugation angles. They concern laminar flows both below and above the onset of the self‐sustained oscillations that precede the transition to turbulence. Fully developed velocity and thermal fields are computed by imposing anti‐periodic conditions at inlet/outlet sections of a single calculation cell. In the range of Reynolds numbers investigated, Nusselt numbers and friction factors first increase with space ratios (up to a value depending on the corrugation angle), then start decreasing with increasing space ratios.

Numerical Evaluation of Fin Performance Under Dehumidifying Conditions

A numerical model of moist air cooling in compact heat exchangers is presented. The model requires the solution of a coupled problem, since interface temperatures, obtained from the solution of the energy equation in adjacent fluid and solid regions, are used to set the appropriate boundary conditions for the transport equation of water vapor in moist air. The approach is completely general, even if the finite-element method is used for the simulations reported in the paper. The numerical results are favorably compared with the corresponding experimental results concerning the rectangular and wavy fins under dehumidifying conditions.

Three-dimensional analysis of convection in two-dimensional wavy channels

Pressure drop and heat transfer characteristics are investigated in the fully developed region of wavy channels that are nominally two dimensional. Three-dimensional (3D) simulations of flow and temperature fields indicate that longitudinal vortices appear as soon as transverse vortex shedding processes begin. In this case, the three dimensionality of the flow is not due to the action of external boundaries, but arises from intrinsic fluid dynamical instabilities. Numerical computations show that the onset of 3D effects has a significant influence on overall friction factors and Nusselt numbers.

Temperature-dependent viscosity and viscous dissipation effects in simultaneously developing flows in microchannels with convective boundary conditions

The effects of viscous dissipation and temperature dependent viscosity in simultaneously developing laminar flows of liquids in straight microchannels are studied with reference to convective boundary conditions. Two different geometries, namely the circular tube and the parallel plate channel, are considered. Viscosity is assumed to vary with temperature according to an exponential relation, while the other fluid properties are held constant. A finite element procedure, based on a projection algorithm, is employed for the step-by-step solution of the parabolized momentum and energy equations. Axial distributions of the local overall Nusselt number and of the apparent Fanning friction factor are presented with reference to both heating and cooling conditions for two different values of the Biot number. Examples of radial temperature profiles at different axial locations and of axial distributions of centerline velocity and temperature are also shown.

Temperature-Dependent Viscosity and Viscous Dissipation Effects in Microchannel Flows With Uniform Wall Heat Flux

A parametric investigation is carried out on the effects of temperature-dependent viscosity and viscous dissipation in simultaneously developing laminar flows of liquids in straight microchannels of constant cross sections. Reference is made to fluid heating conditions with a uniform heat flux imposed on the walls of the microchannels. Six different cross sectional geometries are considered, chosen among those usually adopted for microchannels (circular, flat, square, rectangular, trapezoidal, and hexagonal). Viscosity is assumed to vary with temperature according to an exponential relation, while the other fluid properties are held constant. A finite-element procedure is employed for the solution of the parabolized momentum and energy equations. Due to the high value of the ratio between the length and the hydraulic diameter in microchannels, such an approach is very advantageous with respect to the one based on the steady-state solution of the elliptic form of the governing equations in a three-dimensional domain corresponding to the whole microchannel. Computed axial distributions of the local Nusselt number and of the apparent Fanning friction factor are presented. Numerical results confirm that, in the laminar forced convection in the entrance region of straight microchannels, the effects of temperature-dependent viscosity and viscous dissipation cannot be neglected in a wide range of operative conditions.

FEM for the 3-D analysis of conjugate conduction-convection heat transfer in cross-flow micro heat exchangers

Purpose – The purpose of this paper is to develop a simplified but accurate finite element procedure for the analysis of the conjugate conduction-convection heat transfer in cross-flow micro heat exchangers. Design/methodology/approach – The velocity fields in single microchannels are calculated by solving the parabolised form of the momentum equations and later mapped onto the three-dimensional grid, corresponding to an appropriate portion of the micro heat exchanger, which is used for the solution of the energy equation in its elliptic form. To allow the use of finite elements elongated in the flow direction, layers of perpendicular microchannels can be meshed independently with grids that do not match at the common interface (domain decomposition). Findings – An original and easy-to-implement method has been developed to deal with non-matching grids. Computed results show that increasing the number of microchannels per layer yields relative pressure drop increments that are larger than those displayed ...

Modeling of conjugate conduction and heat and mass convection in tube‐fin exchangers

Purpose – The purpose of this paper is to examine the modeling of simultaneous heat and mass transfer under dehumidifying conditions. Moist air cooling in tube‐fin exchangers is investigated using a finite element technique.Design/methodology/approach – The model requires the solution of a conjugate problem, since interface temperatures must be calculated at the same time as temperature distributions in adjacent fluid and solid regions. The energy equation is solved in the whole domain, including the solid region, and the latent heat flux on the surfaces where condensation takes place is taken into account by means of an additional internal boundary condition.Findings – Thermal performances for different Reynolds numbers of a typical two‐row tube‐fin exchanger are numerically analysed, for both in‐line and staggered arrangements of tubes. The results justify the great importance that the ratio between latent and overall rates of heat transfer has in the design of compact heat exchangers.Practical implicat...

Latent and sensible heat transfer in air‐cooling applications

Purpose – Joint descriptions of both heat and mass transfer and thermodynamic aspects of air‐cooling applications cannot be easily found in the literature. Numerical analyses are a notable exception since suitable physical models and realistic boundary conditions are a prerequisite of accurate simulations. Thus, it is believed that the experience gained with numerical simulations might be of some help also to designers of air‐conditioning and drying systems. This paper seeks to address this issue.Design/methodology/approach – In the text, the physical implications of governing equations and boundary conditions utilized in numerical simulations are extensively discussed. Particular attention is paid to the thermodynamically consistent definition of latent and sensible heat loads, and to the correct formulation of the heat and mass transfer analogy.Findings – Comparisons of analytical and numerical results concerning forced flows of humid air over a cooled plate validate the assumptions made in numerical si...