Jarosław Mikielewicz

Semi-empirical method of determining the heat-transfer coefficient for subcooled, saturated boiling in a channel

Abstract On the assumption that heat transfer during flow boiling is characterized by the resultant dissipation of the energy of convective flow of a two-phase mixture and of vapour bubble generation, a relation has been derived defining the heat-transfer coefficient for a subcooled and saturated boiling in a channel. The relation obtained has been compared with the results of experiments of the present and other authors.

A Common Method for Calculation of Flow Boiling and Flow Condensation Heat Transfer Coefficients in Minichannels With Account of Nonadiabatic Effects

Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena; however, their description with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where bubble generation is not present. A similar picture holds for the case of inside tube condensation, where annular flow structure predominates. In such a case the heat transfer coefficient is primarily dependent on the convective mechanism. In this article a method developed earlier by the authors is applied to calculations of the heat transfer coefficient for flow condensation. The modifications of interface shear stresses between flow boiling and flow condensation are considered through incorporation of the so-called blowing parameter, which differentiates between these two modes of heat transfer. Satisfactory consistency with well-established correlations for condensation has been found, as well as with selected experimental data.

Natural Circulation in Single and Two Phase Thermosyphon Loop with Conventional Tubes and Minichannels

The primary function of a natural circulation loop (i.e. thermosyphon loop) is to transport heat from a source to a sink. Fluid flow in a thermosyphon loop is created by the buoyancy forces that evolve from the density gradients induced by temperature differences in the heating and cooling sections of the loop. An advanced thermosyphon loop consists of the evaporator, where the working liquid boils; and the condenser, where the vapour condenses back to liquid; the riser and the downcomer connect these two exchangers. Heat is transferred as the vaporization heat from the evaporator to the condenser. The thermosyphon is a passive heat transfer device, which makes use of gravity for returning the liquid to the evaporator. Thermosyphons are less expensive than other cooling devices because they feature no pump. There are numerous engineering applications for thermosyphon loops such as, for example, solar water heaters, thermosyphon reboilers, geothermal systems, nuclear power plants, emergency cooling systems in nuclear reactor cores, electrical machine rotor cooling, gas turbine blade cooling, thermal diodes and electronic device cooling. The thermal diode is based on natural circulation of the fluid around the closed-loop thermosyphon (Bielinski & Mikielewicz, 1995, 2001), (Chen, 1998). The closed-loop thermosyphon is also known as a “liquid fin” (Madejski & Mikielewicz, 1971). Many researchers focused their attention on the single-phase loop thermosyphons with conventional tubes, and the toroidal and the rectangular geometry of the loop. For example, Zvirin (Zvirin, 1981) presented results of theoretical and experimental studies concerned with natural circulation loops, and modeling methods describing steady state flows, transient and stability characteristics. Greif (Greif, 1988) reviewed basic experimental and theoretical work on natural circulation loops. Misale (Misale et al., 2007) reports an experimental investigations related to rectangular single-phase natural circulation mini-loop. Ramos (Ramos et al., 1985) performed the theoretical study of the steady state flow in the two-phase thermosyphon loop with conventional tube. Vijayan (Vijayan et al., 2005) compared the dynamic behaviour of the single-phase and two-phase thermosyphon loop with conventional tube and the different displacement of heater and cooler. The researcher found that the most stable configuration of the thermosyphon loop with conventional tube is the one with both vertical cooler and heater.

A theoretical and experimental investigation of direct-contact condensation on a liquid layer

Direct-contact condensation of vapor on a liquid film is an important engineering problem in the chemical and nuclear industries. It is difficult to properly understand the transport processes across the vapor-liquid interface. Whereas previously it was assumed that there is no turbulent eddy diffusion across the interface, more recent evidence suggests that the diffusivity is nonzero. In this study, both analytical and experimental data that confirm a positive value for the diffusivity are presented.

Heat transfer in a channel at supercritical pressure

Abstract The paper concerns a theoretical analysis of the convective heat transfer at supercritical pressures in a channel. The analysis is based on the flow division into two zones with averaged properties the interface between them being the surface of the pseudocritical temperature. Hence, the conservation equations of momentum and energy may be solved separately resulting in analytic formulas for the velocity and temperature profiles and the Nusselt number. The theoretical results are plotted against own experiments with CO 2 revealing a fairly good agreement.

A simplified model of the boiling crisis

Abstract An analytic model of flow boiling crisis in annular flow (at high vapor quality) and our experimental results of Freon 21 are presented in the paper. The model is based on the analysis of the film drying process on the channel wall, thus expressing the mass balance of both the film and the core by means of differential equations. The solution of these equations contains the parameters determined experimentally since their theoretical prediction involves serious difficulties.

Simple theoretical approach to direct-contact condensation on subcooled liquid film

Abstract This work presents an approximate theoretical solution of the direct-contact condensation problem. The analysis is based on a simplified energy equation, in which both velocity and temperature gradients in the direction of the liquid flow are taken from the average energy balance. The theoretical results obtained were compared with experimental results from other authors. Satisfactory agreement was obtained.

Analytical Model With Nonadiabatic Effects for Pressure Drop and Heat Transfer During Boiling and Condensation Flows in Conventional Channels and Minichannels

In this paper a method developed earlier by the authors is applied to calculations of pressure drop and heat transfer coefficient for boiling flow and condensation flow with account of nonadiabatic effects for some recent data collected from the literature. The first effect, the modification of interface shear stresses in an annular flow pattern, is considered through incorporation of the so-called “blowing parameter.” The mechanism of modification of shear stresses at the vapor–liquid interface for such case is presented in detail in the paper. In the case of annular flow it contributes to thickening and thinning of the liquid film, which correspond to condensation and boiling, respectively. There is also another influence of the wall heat flux, where it is influencing the bubble nucleation in the case of the bubbly flow pattern. As a result, a modified general form of the two-phase flow multiplier, applicable both to boiling flow and condensation flow, is obtained, in which the nonadiabatic effects are clearly pronounced. The obtained model of the two-phase multiplier, incorporating the nonadiabatic effects, is additionally used in predictions of the heat transfer coefficient.

An analysis of rivulet formation during flow of an air/water mist across a heated cylinder

Abstract Experimental and analytical investigations of rivulet formation on a heated circular cylinder in an air-water mist flow are performed. The primary objective of the experimental part of the research is to obtain data and observations to help formulate and test the analytical model which is based on the small perturbation theory. The effect of approach velocity and mist quality on the rivulet number is examined

Analysis of Heat Transfer and Fluid Flow in Two-Phase Thermosyphon Loop with Minichannels

The present paper offers an analysis of heat transfer and fluid flow in two phase thermosyphon loop with minichannels. A one-dimensional model of two-phase flow and heat transfer in a closed thermosyphon loop with minichannels was examined. The created general model is based on mass, momentum, and energy balances in the evaporators, rising tube, condensers and the falling tube. The separate two-phase flow model is used in calculations. The numerical results obtained for the selected heater and cooler using the general model of thermosyphon loop indicate that the mass flux increases with increasing length of the heated section and decreases with increasing length of the cooled section of the loop. It was found that the heat transfer coefficient for flow boiling and flow condensation in the steady state increases with increasing heat flux in the heater and cooler with minichannels, respectively. The design and configuration of heaters and coolers has a considerable impact on the efficiency of thermosyphon loop. These factors make it possible to optimize the computer processor cooling.