Giulio Croce

Numerical simulation of roughness effect on microchannel heat transfer and pressure drop in laminar flow

Roughness effects on the heat transfer and pressure losses in microscale tubes and channels are investigated through a finite element CFD code. Surface roughness is explicitly modelled through a set of randomly generated peaks along the ideal smooth surface. Different peak shapes and distributions are considered; geometrical parameters are representative of tubes in the diameter range from 50 to 150 μm. The use of a sufficiently fine mesh allows the direct computation of tube performances under the assumption of incompressible, fully developed flow; a comparison with the predictions of simplified models is also presented

CONVECTIVE HEAT AND MASS TRANSFER IN TUBE-FIN EXCHANGERS UNDER DEHUMIDIFYING CONDITIONS

The article analyzes convective heat and mass transfer in the flow passages of tube-fin exchangers, adopting a simplified two-dimensional approach. The flow structure on the airside of these devices is spatially periodic, with fully developed conditions prevailing a short distance from the entrance. In numerical simulations, symmetric and/or antisymmetric periodicity in pressure, velocity components, temperature, and mass concentration of the water vapour are taken into account to reduce the computational domain. Using a finite-element discretization velocity, temperature and mass concentration fields are computed within wavy, offset-strip, and louver fin surfaces. Quantitative results are also obtained for friction factors, Nusselt numbers, and Colburn factors for heat and mass transfer.

CHT3D: FENSAP-ICE CONJUGATE HEAT TRANSFER COMPUTATIONS WITH DROPLET IMPINGEMENT AND RUNBACK EFFECTS

A conjugate heat transfer procedure including droplet impingement and runback effects is presented and validated against experimental data in a mist-flow heat exchanger configuration and an engine nacelle geometry. Computations include the solution of the air flow field, the prediction of water droplets motion and the evaluation of the cooling effect of the water film on the solid surface. The entire analysis is carried out using FENSAP-ICE (Finite Element Navier-Stokes Analysis Package for Inflight icing), a simulation system developed by Newmerical Technologies for icing applications. The numerical model is described, including the Navier-Stokes solution, the water thin film computation, the droplet impingement prediction and the conjugate heat transfer procedure. The predictions are verified against experimental data for different droplet mass flow rates, showing satisfactory agreement and allowing a useful insight in the physical characteristics of the problem. NOMENCLATURE CD Air-droplet drag coefficient Cp Specific heat [kJ/kgK] D Tube diameter [m] d Droplet diameter [m] G Droplet mass flow rate [kg/mh] h Heat transfer coefficient [kW/m K] HQ Total enthalpy [kJ/kg] K Droplet inertia parameter k Thermal conductivity [kW/mK] L Reference length [m] raj Impinging mass flow [kg/s] t Associate Fellow. AIAA Copyright ©2002 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., by permission mev Evaporating mass flow [kg/s] rrifin Incoming film mass flow [kg/s] M Molecular weight [kg/kmol] mev Evaporating mass flow [kg/s] N Finite element shape function p Static pressure [Pa] Pr Prandtl number q Specific heat flux [kW/m] Qa Conduction heat flux from the wall [kW] Qa Convective heat flux to the air [kW] Red Droplet Reynolds number Sc Schmidt number T Static temperature [K] Tf Film temperature [K] Tfin Incoming film temperature [K] Td Droplet temperature [K] Tr Kinetic heat contribution for droplets [K] UOQ Reference air velocity [m/s] v, Vi Air velocity vector and i-th component [m/s] v, v\ Droplet velocity vector and i-th component [m/s] X, Y Dimensional spatial coordinates [m] x, y Non-dimensional spatial coordinates a Liquid volume fraction /3 Collection efficiency 6ij Kronecker delta A Latent heat of evaporation [kJ/kg] ^ Dynamic viscosity [kg/ms] Q/, Q, Fluid and solid domain p Air density [kg/m] F Solid Fluid interface Tij Shear stress tensor component [N/m]

Numerical analysis of forced convection in plate and frame heat exchangers

A three‐dimensional numerical investigation of flow field and heat transfer in sine‐wave crossed ducts is presented. Numerical simulations are carried out using a finite element procedure based on an algorithm which shares many features with the SIMPLER finite‐volume method, and utilizes equal order pressure–velocity interpolation functions. Since the flow, after a short entrance regime, reaches the fully developed condition, the computational domain can be reduced to a single periodic element and periodic boundary conditions are assumed at the entrance, the exit and the sides. The thermal performance and the frictional pressure losses of the crossed‐corrugated plates are investigated for different Reynolds number, from steady up to transitional regimes. The evolution from steady to unsteady flow structure is detected and the influence of the unsteadiness on heat transfer and on pressure drop is analysed. Simulations are performed for both air (Pr=0.7) and water (Pr=7) as the flow medium and the dependence of Nusselt number on Prandtl number is investigated.

Compressibility and Rarefaction Effects on Pressure Drop in Rough Microchannels

A numerical analysis of the flow field in rough microchannel is carried out with a finite volume compressible solver, including generalized Maxwell slip flow boundary conditions suitable for arbitrary geometries. Roughness geometry is modeled as a series of triangular obstructions. The relative roughness from 0% to 2.65% was considered. Because for truly compressible flow we have no fully developed flow condition, the simulation is performed over the whole length of the channel. A wide range of Mach numbers is considered, from nearly incompressible to chocked flow conditions. Flow conditions with a Reynolds number up to around 200 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from a very low value to 0.0249. Performance charts are presented in terms of both average and local Poiseuille number as a function of local Knudsen, Mach, and Reynolds numbers. In particular, it appears that roughness strongly decreases the reduction in pressure loss due to rarefaction. Thus, the roughness effect is stronger at a high Knudsen. Furthermore, the compressibility effect has a major effect on pressure drop when the local Mach number exceeds 0.3.

Numerical simulation of glass fogging and defogging

A numerical procedure for the prediction of fogging and defogging phenomena is presented. The simulation involves the solution of an air flow field along a cold solid surface, the evaluation of the unsteady conduction through the solid itself, and a model for the heat and mass transfer within the thin water layer on the fogged surface. A suite of routines for the unsteady simulation of the water layer evolution is coupled with an equal order finite element Navier Stokes solver and a finite volume conduction code. The procedure is fully independent of the numerical details of the solid and fluid domain solvers. Two different coupling approaches may be followed: A loose one, where the Navier Stokes solution is used only for a steady state estimate of the heat transfer coefficient, or a close one, where the Navier Stokes, conduction and water layer codes are iterated simultaneously. The latter is required for the problem of natural convection, where temperature (and thus the energy balance of the water layer) and flow field are coupled. The water layer is modelled as a collection of closely packed tiny droplets, leaving a portion of dry area among them. The effect of the contact angle is taken into account, and physical assumptions allow to define the local ratio between wet and dry surface for both the fogging and defogging process. As a case study, a comparison with experimental data for a complete fogging and defogging cycle of a glass lens in natural convection is presented.

FENSAP-ICE: Analytical Model for Spatial and Temporal Evolution of In-Flight Icing Roughness

Ice roughness, which has a major influence on in-flight icing heat transfer and, hence, ice shapes, is generally input from empirical correlations to numerical simulations. It is given as uniform in space, while sometimes being varied in time. In this paper, a predictive model for roughness evolution in both space and time during in-flight icing is presented. The distribution is determined mathematically via a Lagrangian model that accounts for the stochastic process of bead nucleation, growth, and coalescence into moving droplets and/or rivulets and/or water film. This general model matches well the spatial and temporal roughness distributions observed in icing tunnel experiments and is embedded in FENSAP-ICE, extending its applicability outside the range of airfoil types for which correlations exist. Thus, an additional important step has been taken toward removing another empirical aspect of in-flight icing simulation.

NUMERICAL SIMULATION OF HEAT TRANSFER IN MIST FLOW

A numerical simulation of heat transfer over a row of tubes, in the presence of mist flow, is described. Computations include the solution of the flow field around the tubes, the prediction of the motion of water droplets, and the evaluation of the cooling effect of the water film on the tube surface. The entire analysis is carried out using FENSAP-ICE (Finite Element Navier-Stokes Analysis Package for In-flight icing), a simulation system developed by Newmerical Technologies for icing applications. The numerical model is described, including the Navier-Stokes solution, the water thin film computation, the droplet impingement prediction, and the conjugate heat transfer procedure. The predictions are verified against experimental data for different droplet mass flow rates, showing satisfactory agreement and allowing a useful insight in the physical characteristics of the problem.

Numerical Investigation of Microflow Over Rough Surfaces: Coupling Approach

A numerical analysis of the flow field in rough microchannel is carried out decomposing the computational physical domain into kinetic and continuum subdomains. Each domain size is determined by the value of a proper threshold parameter, based on the local Knudsen number and local gradients of macroparameters. This switching parameter is computed from a preliminary Navier–Stokes (NS) solution throughout the whole physical domain. The solution is then advanced in time simultaneously in both kinetic and continuum domains: The coupling is achieved by matching half fluxes at the interface of the kinetic and Navier–Stokes domains, taking care of the conservation of momentum, energy, and mass through the interface. The roughness geometry is modeled as a series of triangular obstructions with a relative roughness up to a maximum of 5% of the channel height. A wide range of Mach numbers is considered, from nearly incompressible to chocked flow conditions 0.001 ≤ Ma ≤ 0.75 and a Reynolds number up to 170. To estimate rarefaction effect, the flow at Knudsen number ranging from 0.01 to 0.08 and fixed pressure ratio has been considered. Accuracy and discrepancies between full Navier–Stokes, kinetic, and coupled solutions are discussed, assessing the range of applicability of first order slip condition in rough geometries. The effect of the roughness is discussed via Poiseuille number as a function of local Knudsen and Mach numbers.

A Conjugate Heat Transfer Procedure for Gas Turbine Blades

Abstract: A conjugate heat transfer procedure, allowing for the use of different solvers on the solid and fluid domain(s), is presented. Information exchange between solid and fluid solution is limited to boundary condition values, and this exchange is carried out at any pseudo‐time step. Global convergence rate of the procedure is, thus, of the same order of magnitude of stand‐alone computations.