Lounes Tadrist

A review on boiling heat transfer enhancement with nanofluids

There has been increasing interest of late in nanofluid boiling and its use in heat transfer enhancement. This article covers recent advances in the last decade by researchers in both pool boiling and convective boiling applications, with nanofluids as the working fluid. The available data in the literature is reviewed in terms of enhancements, and degradations in the nucleate boiling heat transfer and critical heat flux. Conflicting data have been presented in the literature on the effect that nanofluids have on the boiling heat-transfer coefficient; however, almost all researchers have noted an enhancement in the critical heat flux during nanofluid boiling. Several researchers have observed nanoparticle deposition at the heater surface, which they have related back to the critical heat flux enhancement.

Numerical investigation of heat and mass transfer of an evaporating sessile drop on a horizontal surface

A convection-diffusion model is developed to analyze the effect of buoyant convection in the surrounding air on the heat and mass transfer phenomena during the evaporation of a pinned water drop deposited on a horizontal substrate of large dimensions. The substrate is maintained at constant temperature which can be equal or higher than the temperature of the ambient air. The mathematical model accounts for the motion of the gas phase surrounding the drop due to thermal and solutal buoyancy effects, while only thermal diffusion is considered in the liquid phase. A quasisteady state regime is adopted because of the slow motion of the liquid-gas interface as well as the induced heat and mass transfer phenomena in both phases. The numerical results obtained with the diffusion model or the convection-diffusion model show that heat and mass transfer rates are important toward the contact line. The heat required for evaporation process is taken from the environment, both the liquid and the gas phase, and results...

Destabilization Mechanisms and Scaling Laws of Convective Boiling in a Minichannel

Convective flow boiling in a minichannel is presently analyzed. A 889-m hydraulic diameter minichannel is used for the experiments conducted at several heat fluxes provided to the minichannel for variable inlet mass flow rates. A local analysis of the two-phase flow behavior through pressure and temperature measurements is detailed in the paper. Based on the flow boiling behavior, a destabilization mechanism is proposed on the vapor plug formation assumption. A theoretical criterion of the two-phase flow destabilization is proposed using a simplified model. The approach is compared with the experimental results. A scaling law is found with a nondimensional analysis of the experimental results.

Heat Transfer Enhancement in Short Corrugated Mini-Tubes

Heat transfer phenomena are studied with standing waves inside the tubes for static and moving sinusoidal corrugated walls. The past studies have been done on big-size (dimensions in m) and micro-sized circular tubes (dimensions in μm). We are focusing on intermediate size tubes (dimensions in mm). Numerical simulations, using finite volume commercial software, were performed to study the effects of spatial wavelengths on heat transfer enhancement and associated pressure drop. We imposed 5, 10, 15 and 20 3D sinusoidal radial sine waves along the length of the tube. Heat transfer characteristics of static corrugated wavy walls were calculated for various imposed Reynolds numbers (1 < Re < 120) and amplitude of the wave was varied from 1 to 20 % of the diameter of the tube. For static wall case, upon increasing the number of sine waves, the Nusselt number starts to decrease; the associated pressure drop and friction factor increases very rapidly at the highest values of amplitude. On the other hand, in comparison to the static corrugated wall tube, the pressure drop is reduced by 20–80 % and heat transfer is enhanced by 35–70 % for highest amplitude when frequencies in the range 0 < f < 60 Hz are imposed on tube wall to make the corrugated tube moving in transverse direction.

Experimental Study of Boiling Heat Transfer on Multiple and Single Nucleation Sites Using a Boiling-Meter

The general objective of this study is to contribute to a better understanding of heat transfer in a nucleate boiling regime. The aim is to determine the heat transfer characteristics under controlled operating conditions (thermodynamics of the fluid, noncondensable gas, surface state). Experimental investigations have been carried out in natural convection and nucleate boiling regimes. An experimental device was realized to perform boiling experiments using a boiling-meter, allowing investigations for different orientations of the wall. The boiling-meter is designed to investigate boiling for single and multiple nucleation sites. The purpose of this paper is to detail the experimental setup as well as the boiling-meter. This device allows the determination of the temporal heat transfer characteristics evolutions. In particular, this new device allows bringing to light the phenomenon of nucleation, growth, and detachment of generated vapor bubbles on a single artificial nucleate site, as well as for multiple natural nucleation sites. First results of the influence of the orientation of the heating wall for multiple and single nucleation sites on heat transfer are presented and analyzed.

Fluid flow inside and outside an evaporating sessile drop

The sessile drop evaporation is a phenomena which is extensively studied in the literature, but the governing effects are far from being well understood especially those involving movements taking place in both liquid and gas phases. The present work numerically studies the flow within and around an evaporating sessile drop. The flow is induced by the strong mass loss at contact line, the thermo-capillary effect and the buoyancy effect in the surrounding air. The results showed that buoyancy-induced flow in gas phase weakly influences thermo-capillarity-induced flow in the liquid phase. Buoyancy effect can strongly modify the temperature distribution at liquid-gas interface and thus the overall evaporation rate of the drop when the substrate is heated.

Multicellular piezoelectric actuator for setting in motion fluids, and heat exchange enhancement

In this paper, a multi-cellular piezoelectric actuator has been dimensioned, designed and integrated into an electro-active morphing wall heat exchange demonstrator. The design study was performed using finite element simulations. Numerical study shows that the obtained deflections vary from 77 to 462 μm for applied displacement ranging from 3 to 18 μm. Once the operating principle validated, a series of actuators has been machined and tested. Dynamic tests (unloaded) gives a maximum displacement about 230 μm for a frequency ranging up to 100 Hz. However the static tests give results with a good agreement with simulation. The developed actuators have been introduced into a demonstrator to show the feasibility of an intensification of heat exchange by electro-active morphing. The recovered data have demonstrated the feasibility of pumping for a traveling wave of 5 Hz closing quasi-totally the channel. Another result is the reduction of wall temperatures during the application of a traveling wave on the, thereby that the heat transfer was intensified. Moreover, an increase in the rate of flow was observed. All this results demonstrate the enhancement of heat transfer.

Co-Current Air-Water Two Phase Flow Patterns in Horizontal and Vertical Circular Tube Under Various Inlet Conditions

An experimental apparatus is setup to analyze a co-current air-water two phase flow in a 3 mm inner diameter tube with horizontal and vertical orientations. Air is axially injected through a nozzle of 260 μm of inner diameter. Air and water flow rates are accurately controlled at the inlet, covering a range of apparent velocities JL = 0.00118 to 0.0786 m/s, JG = 0.002 to 3.538 m/s for the horizontal tube and JL = 0.00078 to 0.0589 m/s, JG = 0.003 to 3.538 m/s for the upward flow. A fast camera with 250 fps is utilized to visualize the flow patterns. The experiments showed that the flow structures are very sensitive to inlet conditions. Within the covered range of velocities, several flow patterns were observed, namely bubbly flow, bubbly-slug transition flow, slug flow, slug-annular transition flow, annular flow, wavy annular flow and annular flow with dry zones. In the bubbly flow regime, a particular bubbly helical flow is observed before the dispersed bubbly flow.Copyright

Experimental Analysis of a Two Phase Air-Water Flow in a Tube of Small Size Diameter Under Various Inlet Conditions

An experimental apparatus is setup to analyze a two phase air-water upward flow in a vertical tube with an inner diameter of 3 mm. Air is axially injected through a microduct of 260 μm inner diameter. Various inlet conditions for air pressure and water flow rate are tested covering a wide range of superficial velocities JL = 0.221 to 0.312 m/s and JG = 0.061 to 0.083 m/s for a given position of air injection (x = 8cm). A fast camera with 250 fps is used to visualize different flow regimes. Experiments showed that the flow type is very sensitive to inlet conditions and several flow regimes were observed namely: the bubbly flow, the slug flow and the annular flow.Copyright

Experimental Investigation of the Effect of the Ambient Gas on Evaporating Sessile Drops

This paper presents the results of an experimental study of evaporating sessile drops in a controlled environment. The experimental setup allowed the investigation of the evaporation rate of sessile drops under reduced pressure (40 to 1000 mbar) and various ambient gases. Sessile drops of initial volume 2.5μL are deposited on substrates and left to evaporate in a controlled atmosphere. The effect of reducing pressure on the evaporation rate as well as changing the ambient gas is studied. Three different gases are used; namely Helium, Nitrogen and Carbon Dioxide. The role of vapour diffusion as a limiting mechanism for evaporation is studied. It is found that in all cases the evaporation rate is limited by the mass diffusion in the ambient gas provided that interfacial conditions are properly accounted for. This includes important evaporative cooling observed at higher evaporation rates and lower substrate thermal conductivity.