Hassan Peerhossaini

Rheology, flow behaviour and heat transfer of ice slurries: a review of the state of the art

Abstract This paper reviews recent studies on rheology, flow behaviour and heat transfer of two-phase aqueous secondary refrigerants (ice slurries). The difficulties in measuring their rheological properties for which standard rheometers prove to be poorly adapted are analysed. Special attention is paid to “vane-in-cup” rheometers, which can make it possible to determine the yield stress. Pressure losses in cylindrical tubes have been measured by many authors. Even if the results generally agree with two-phase flow theory, divergences in sensitivity to the solid fraction can be observed. The stratification observed by some authors for small-Reynolds-number flows and its effects on the pressure drop are addressed. In a last part, information concerning numerical values of the heat transfer coefficient of ice slurries are summarized. Heat transfer coefficients depend on many parameters, but are largely influenced by the flow regime. A geometry of heat exchanger is then proposed, which has already been effective for single-phase flows, and which may enhance heat transfer in ice slurry flows.

Droplets formation in turbulent mixing of two immiscible fluids in a new type of static mixer

The emulsification process in a static mixer high-efficiency vortex in turbulent flow is investigated. This new type of mixer generates coherent large-scale structures, enhancing momentum transfer in the bulk flow and hence providing favourable conditions for phase dispersion. The generation of the emulsion is described via a classical size-distribution function characterised by the Sauter diameter and a dispersion factor.

A thermal model for prediction of the Nusselt number in a pipe with chaotic flow

It is currently well established that Lagrangian chaos intensifies heat transfer significantly [J. Fluid Mech. 209 (1989) 335]. It thus appears to be a promising technique for the design of compact, high-performance heat exchangers and heat exchanger-reactors. However, the design of such apparatus requires extensive calculations. The objective of this work is to implement a simplified thermal model with which to simulate heat transfer in a twisted pipe (of a shell and tube heat exchanger) of two tube configurations, helically coiled or chaotic, without requiring the heavy calculations needed in the numerical resolution of the Navier–Stokes and energy equations. The large database obtained from the parametric study of the variation of the Nusselt number using the heat transfer model developed here, allows one to correlate Nu with Re, Pr, Nbends: Nu=1.045Re0.303Pr0.287Nbends−0.033. This correlation is valid for coil geometry with alternating planes of curvature, i.e. chaotic configuration and the range of validity of the correlation is Re∈[100;300], Pr∈[30;100] and Nbends∈[3;13].

A Numerical Study of Dean Instability in Non-Newtonian Fluids

We present a numerical study of Dean instability for non-Newtonian fluids in a laminar 180deg curved-channel flow of rectangular cross section. A methodology based on the Papanastasiou model (Papanastasiou, T. C., 1987, J. Rheol., 31(5), pp. 385–404) was developed to take into account the Bingham-type rheological behavior. After validation of the numerical methodology, simulations were carried out (using FLUENT CFD code) for Newtonian and non-Newtonian fluids in curved channels of square or rectangular cross section and for a large aspect and curvature ratios. A criterion based on the axial velocity gradient was defined to detect the instability threshold. This criterion was used to optimize the grid geometry. The effects of curvature and aspect ratio on the Dean instability are studied for all fluids, Newtonian and non-Newtonian. In particular, we show that the critical value of the Dean number decreases with increasing curvature ratio. The variation of the critical Dean number with aspect ratio is less regular. The results are compared to those for Newtonian fluids to emphasize the effect of the power-law index and the Bingham number. The onset of Dean instability is delayed with increasing power-law index. The same delay is observed in Bingham fluids when the Bingham number is increased.

Temperature and Heat Flux Behavior of Complex Flows in Car Underhood Compartment

In this work heat transfer and temperature behavior of complex flows encountered in the vehicle underhood compartment is experimentally studied and described with simple models. Underhood thermal measurements made on a passenger vehicle in a large-scale wind tunnel are reported here. The underhood is instrumented by 80 surface and air thermocouples and 20 fluxmeters. Measurements are carried out at three thermal functioning points, in all of which the engine is in operation and the front wheels are positioned on the test facility with power-absorption-controlled rollers. Models are proposed to predict the maximum temperatures and time constants of the underhood components as functions of the car speed and car engine power. The relative errors of the models are 3.6% and 3.7%, respectively. The maximum temperature and the time constant are crucial in the design and optimization of the underhood aerothermal management system. The results obtained in the present work also provide a large database for validation of numerical codes dealing with underhood cooling management.

Thermal and Hydrodynamic Performances of Chaotic Mini-Channel: Application to the Fuel Cell Cooling

Currently, heat exchangers allowing the thermal management of low-temperature fuel cells (PEMFC) are integrated in the bipolar plates and are constituted of a network of straight channels. The flow regime is laminar and thus unfavorable to intense convective heat transfer. In order to increase the power density of the fuel cells, the use of chaotic geometries in the cooling system is envisaged to promote high convective heat transfer. In the present study, several chaotic three-dimensional mini-channels of rectangular cross-section (2 millimeters × 1 millimeter) are evaluated in terms of heat transfer efficiency, mixing properties, and pressure losses. Their performances are compared both to those of the straight channel geometry currently used in the cooling systems of the PEMFC and those of a square-wave mixer. Two Reynolds numbers are considered: 100 and 200. It is shown that a 3-D chaotic channel geometry significantly improves convective heat transfer over that of regular straight or square-wave mixer channels. Of all the geometries studied, one induces higher heat transfer intensification (mean Nusselt number equal to 20) with a strong pressure loss. With an alternative geometry, a better compromise between heat transfer and pressure loss is obtained. However, all of the chaotic geometries present similar mixing rate for the two Reynolds numbers studied.

Numerical and experimental investigation of direct electric conduction in a channel flow

Heat generation by direct electric conduction in a fully developed channel flow was studied to clarify interaction between the hydrodynamic, electric and thermal phenomena involved. The equations governing the system were solved numerically by a finite volume code. It was found that the difference between the residence time of the fluid flowing close to the wall and in the center of the channel causes deformation of the velocity profiles. This phenomenon creates a competition between the fluid particles flowing in these two regions and results in equalizing the temperature and velocity distribution in the channel span. An experimental investigation carried out in parallel with the numerical study yielded good agreement. An array of longitudinal streaks found on the channel electrode wall, with LIF visualization, is attributed to an instability phenomenon caused by electric body force. The wavelength of the streaks was measured and the control parameters of the instability were determined.

Active control of natural convection in a fluid layer with volume heat dissipation

Abstract A fluid layer subjected to an internal heating source and cooled from above and below is studied. Using linear stability analysis and numerical simulation it is shown that the critical Rayleigh number related to the bifurcation from the motionless conductive state to a convective state can be increased by controlling the heating power. A feedback-control strategy using the deviation of the real fluid temperature from that of the associated conductive state is applied for this purpose.

Eddy Heat Transfer by Secondary Görtler Instability

Experimental measurements of flow and heat transfer in a concave surface boundary layer in the presence of streamwise counter-rotating Gortler vortices show conclusively that local surface heat-transfer rates can exceed that of the turbulent flat-plate boundary layer even in the absence of turbulence. We have observed unexpected heat-transfer behavior in a laminar boundary layer on a concave wall even at low nominal velocity, a configuration not studied in the literature: The heat-transfer enhancement is extremely high, well above that corresponding to a turbulent boundary layer on a flat plate. To quantify the effect of freestream velocity on heat-transfer intensification, two criteria are defined for the growth of the Gortler instability: P z for primary instability and P rms for the secondary instability. The evolution of these criteria along the concave surface boundary layer clearly shows that the secondary instability grows faster than the primary instability. Measurements show that beyond a certain distance the heat-transfer enhancement is basically correlated with P rms , so that the high heat-transfer intensification at low freestream velocities is due to the high growth rate of the secondary instability. The relative heat-transfer enhancement seems to be independent of the nominal velocity (global Reynolds number) and allows predicting the influence of the Gortler instabilities in a large variety of situations.

Effects of Car Inclination on Air Flow and Aerothermal Behavior in the Underhood Compartment

The study presented here concerns the impact of car inclination on the temperatures in the vehicle underhood compartment. We report here underhood thermal measurements carried out on a vehicle in wind tunnel S4 of Saint-Cyr. The underhood is instrumented by 80 surface and air thermocouples. Measurements are carried out for three thermal functioning points: the engine in operation and the front wheels positioned on the test facility with power-absorption-controlled rollers. Three car inclinations are tested.© 2008 ASME