Marc Miscevic

About the use of fibrous materials in compact heat exchangers

Abstract One way of increasing the exchange surface area of a conventional exchanger is to replace the fins by a porous structure. Under these conditions, the exchange surface area can reach very high values. However, this increase amplifies the pressure drop of the fluid circulating in the porous matrix. It is therefore necessary to determine the optimal parameters of the porous medium in order to maximize the heat transfer with regard to the pressure drop. The characteristics of two kinds of fibrous materials are discussed: randomly stacked fibers and metallic foams that are used in industrial systems. The randomly stacked fibrous materials present variations of porosity and permeability depending on fiber diameter and aspect ratio. The metal foams are commercial products made with aluminium that present very high porosities and permeabilities due to the arrangement of the solid matrix. The transport properties are analyzed for both materials: permeability, friction factors and effective thermal conductivity. Local and global Nusselt numbers as well as the entrance length are derived from experimental data obtained in rectangular sintered fibrous channels and in cross-flow heat exchangers. Finally, several aspects of two-phase flow case (i.e. liquid–vapor) are discussed.

An experimental and theoretical investigation of the transient behavior of a two-phase closed thermosyphon

This paper presents an experimental and theoretical investigation of the two-phase closed thermosyphon (TPCT) behavior in transient regimes. Experimental results show two kinds of TPCT response. We focus on regular variations of operating system variables, where a mathematical model has been developed in order to obtain an analytical expression of the system response time. The dependence of this response time according to the various parameters is linked to geometry and heat transfer laws. The model can be considered as a simple and efficient tool for designing TPCTs in both transient and steady regimes.

Capillary Pumped Loop Startup: Effects of the Wick Fit on Boiling Incipience

The onset of boiling in a capillary pumped loop evaporator is experimentally studied. The evaporator of the system contained a porous material in which the working fluid (n-pentane) was vaporized. The system was started up with the evaporator completely flooded, and boiling was initiated by application of a heat load. A specific flat plate evaporator was designed where the distance between the heating plate and the wick can be controlled. The two parameters studied were confinement and power applied (600 W maximum). The superheat needed to initiate boiling was strongly dependent on the quality of the contact between the heating plate and the wick and on the power applied. The initial superheat played a decisive role in the behavior of the system at startup. From the beginning, there were strong couplings between the physical phenomena occurring in the evaporator and the reservoir.

Heat and Mass Transfer in Capillary-Pumped Liquid Films in Square Cross-Section Minichannels

This study addresses heat and mass transfer during the vaporization of a liquid in a heated square cross-section mini-channel. A theoretical model is developed in steady state using the radius of curvature as a variable. One-dimensional simulations have been performed. An analysis of this model reveals that heat and mass transfer is governed by two main groups of non-dimensional numbers (i.e., Reynolds × Boiling and Weber × Boiling2 numbers). Maps of heat transfer performance are thus proposed according to these non-dimensional numbers. A reduced model is finally derived, allowing the main parameters to be expressed (such as the extended meniscus length) analytically.

Boiling incipience of highly wetting liquids in horizontal confined space

Abstract This study concerns boiling incipience of highly wetting liquids (n-pentane), in transient conditions, in horizontal confined space between a vertical heating cylinder and a disk. The two controlled parameters were the heat flux, ranging from 50 to 200 kWxa0m−2 and the thickness of the liquid in the confined space, ranging from 0 to 240 μm. Experiments were conducted with both smooth and porous disks. The results suggest that in the porous-disk configuration, boiling could appear on the non-heated wall.

Complete Convective Condensation Inside Small Diameter Horizontal Tubes

An experimental study of complete convective condensation inside narrow channels is presented in this paper. Two-phase flows patterns and their transition (annular, annular-wavy, slug and bubbly flow) are visualized for the two tube diameters under study. A significant difference is observed for the two sizes of tube. Experimental results of the bubble radius decrease are then determined and compared to a model of bubble collapse in a subcooled and infinite liquid.Copyright

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 Capillary Pumping During Vaporization in a Square Cross Section Microchannel

An experimental study is undertaken to investigate microcapillary pumped loop (micro-CPL) evaporator behaviours. The experimental set-up is made of a horizontal square cross section glass microchannel which is locally heated up in order to analyze vaporization. A specific procedure allowed us to investigate the capillary pumping due to vaporization: the mass flow rate according to heat flux applied is determined. A laser-induced fluorescence technique has been used to observe phases distribution in the microchannel during vaporization, especially the liquid raising in the corners. The extended meniscus zone length has been determined as a function of the heat flux using image processing.Copyright