Paola D'Agaro

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

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.

Numerical simulation of windshield defogging process

Abstract A suite of routines for the prediction of environment moist condensation and evaporation on solid surfaces is presented. The physical problem requires the solution of the airflow field along a (cold) solid surface, the evaluation of the unsteady conduction through the solid itself, and the development of a suitable model for the heat and mass transfer through the thin water layer on the fogged surface. The routines for the unsteady simulation of the water layer development are designed as a purely interfacial procedure, minimizing the exchange of information with both the flow and the conductive solver. This allows the coupling with different solvers. Here, the model is used in connection with a commercial computational fluid dynamics solver, in order to predict the defogging process of a car windshield. 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 the local ratio of the wet surface to the dry surface to be defined for both the fogging and the defogging process.

Computational Analysis of Conjugate Heat Transfer in Gaseous Microchannels

A fully conjugate heat transfer analysis of gaseous flow in short microchannels is presented. Navier–Stokes equations, coupled with Maxwell and Smoluchowski slip and temperature jump boundary conditions, are used for numerical analysis. Results are presented in terms of Nusselt number, heat sink thermal resistance, and resulting wall temperature as well as Mach number profiles for different flow conditions. The comparative importance of wall conduction, rarefaction, and compressibility are discussed. It was found that compressibility plays a major role. Although a significant penalization in the Nusselt number, due to conjugate heat transfer effect, is observed even for a small value of solid conductivity, the performances in terms of heat sink efficiency are essentially a function only of the Mach number.

Droplet Buildup and Water Retention Prediction in Condensation Processes

A numerical model for the prediction of the buildup of a moist air condensate layer is presented. The model simulates the process of birth, growth, coalescence, and possible motion of each individual condensate droplet. Due to its computational intensity, such an approach is not feasible for the simulation of complex configurations of industrial interest, but allows for the identification of the major parameters influencing the water layer build up process. In particular, the model requires, as input data, the values of droplet advancing and receding contact angles, and computes the unsteady evolution of the droplet distribution. The droplet movement, as well as the moving droplet velocity, is detected by a force balance. Proper average values of practical interest, such as wetted area and water retention, are evaluated and compared with published experimental data. Such average data can be used as calibration values for global integral parameters usually required in order to incorporate the water layer effect within CFD computations in more complex geometries. The results show a good agreement with the experimental data and confirm that our model, although based on a small number of independent parameters (essentially the contact angles) and a relatively simple schematization of the complex physics behind the droplet history, predicts the water retention well within engineering accuracy. In particular, the hysteresis between advancing and receding angles seems the dominant effect.

Thin Film Breakup and Rivulet Evolution Modeling

The present paper is aimed at the modeling of a continuous film breakup into individual rivulets, leading to the formation of dry patches on the substrate surface. Following an approach already successfully applied to the prediction of still/moving droplet configuration, we attempt to model the details of a single possible film breakup and its evolution over a two-dimensional domain via a phenomenological model. Based on the momentum, energy and mass flow balance of the capillary ridge on the border of the dry patch, the proposed model is validated against both numerical prediction and experimental results from the open literature. Such a detailed prediction may not be practical for the simulation of complex geometrical configuration (which may include, as an example, multiple breakups on the surface of a the whole aircraft subject to icing condition), but can be used to look for statistically significant parameters that can be used to provide proper boundary conditions for fully 3D CFD computations.