Marian Trela

Enhancement of condensation heat transfer by means of EHD condensate drainage

The problem of electrohydrodynamic (EHD) condensation enhancement has been studied very recently. The electric field is commonly generated in such case by means of rod and mesh electrodes. The EHD method is suitable for dielectric media used in refrigeration and heat pump devices. The strong positive EHD effect has been obtained so far for vertical and horizontal bare tubes. However it was found that this effect for horizontal integral-fin tubes, commonly used in heat exchangers, is negligible small. The mechanism of the EHD condensation enhancement is discussed in the paper and a novel arrangement of the tube-electrode system is proposed. It consists of a horizontal finned tube with a rod type electrode placed beneath the tube. The experimental investigations have been carried out for this tube-electrode arrangement using HCFC-123 as a working medium. The obtained results confirmed the expectations since the application of EHD method for the new tube-electrode configuration increased the heat transfer coefficient from 27% to 110%, depending on the electrode potential. A simple model of the EHD condensate drainage has been proposed for the new tube-electrode arrangement.

Enhancement of condensation heat transfer by means of passive and active condensate drainage techniques

A short state-of-the-art review on the passive and active enhancement of condensation heat transfer techniques developed recently is presented in the paper. The particular attention is paid to the methods involving the augmentation of the condensate drainage. As an example of the passive technique the method of condensate drainage enhancement by using the drainage strip is presented. For an active method of heat transfer enhancement a novel EHD technique is described. For both methods the own experimental results as well as theoretical models are provided.

An approximate calculation of heat transfer during flow of an air-water mist along a heated flat plate

Abstract An approximate calculation of heat transfer during laminar flow of mist air along a flat plate placed in a channel is carried out. The high heat flux is shown to result from the superposition of film evaporation process from the plate surface and convective heat transfer. Convective heat flux appears higher than during a dry air flow due to the enhancement effect of the droplets. Theoretical results show good agreement with experiments.

Enhancement of condensate drainage from a horizontal integral-fin tube by means of a solid strip

Abstract This paper describes the process of intensification of condensation heat transfer on a single horizontal integral-fin tube due to the enhancement of condensate drainage. An analytical model of the drainage effect of a special longitudinal solid strip is presented. The so-called drainage parameter seems to be the most important in the model. Exemplary calculations have been made to investigate the effect of tube and strip geometry on the enhancement of the drainage process. The experimental investigations of the condensate drainage from a single horizontal integral-fin and unfinned tube by using a longitudinal solid drainage strip are presented in this paper. The condensation heat-transfer augmentation in the range 6–25% was achieved by means of a solid drainage strip.

Analysis of application of feed-water injector heaters to steam power plants

Analysis of application of feed-water injector heaters to steam power plants Steam-water injectors are devices in which exchange of mass, momentum and energy between two fluids being in direct contact, occurs. They can operate as pumps, mixers or direct contact heat exchangers. In the last aspect their use as feed-water heaters in Rankine thermal cycle of steam power plants both in land and sea applications (to merchant and naval ships) is very interesting. This paper presents selected results of heat-and-flow investigations of a supercritical steam-water injector, obtained in Institute of Fluid Flow Machinery, Polish Academy of Sciences (IMP PAN). On their basis value of average heat transfer coefficient for mixing chamber was determined; the obtained values were even a few dozens greater than those for classical shell-and-tube heaters. In the theoretical part of this work is presented an original injector model based on balances of mass, momentum and energy, written for control volumes containing separately particular elements of injector. On the basis of the model flow parameters in characteristic cross-sections of injector were determined. The calulations were performed for two different injectors tested in IMP PAN (Gdańsk) and SIET (Piacenza, Italy), and their good compliance with experimental data was achieved.

A semi-theoretical model of stability of vertical falling liquid films

Abstract A semi-theoretical model of minimum wetting rates for gravity-driven liquid films under isothermal conditions is presented. It is based on the experimental observations that disturbed liquid layers break up more easily than smooth ones and that the contact angle is a variable quantity. The theory comprises the main features of the three classical models used so far: flow distortion, force balance considerations, minimum-energy requirement. Some experimental evidence supports the main ideas of the model. There is a reasonably good agreement with respect to the boundaries of the transition region.

Minimum wetting rate for a decelerating liquid film

Abstract The paper presents an analytical study of a laminar decelerating liquid film falling along a vertical plate. Approximate solutions are obtained for the boundary layer within the film, film thickness, entrance length, and minimum wetting rate. It is shown that the analysis may be extended also to a film flowing over a horizontal cylinder. The theory is in reasonable agreement with the parametric trends observed in experiments on horizontal cylinders.

Analysis of fouling thermal resistance of feed-water heaters in steam power plants

Analysis of fouling thermal resistance of feed-water heaters in steam power plants In this paper a problem is discussed of thermal degradation of shell-and-tube recuperative feed-water heaters due to heat transfer surface fouling. Application (to an example steam cycle of power plant) of DIAGAR software system intended for the analysing of impact of degradation of a particular heat exchanger on whole recuperative system performance, is presented. On the basis of the systematically performed simulative calculations it was concluded that the degradation of feed-water heaters of the lowest extraction pressure results in an additional load imposed on the heaters of higher extraction pressures, moreover it was estimated that the degradation of the heaters due to presence of sediments of the thermal resistance reaching 5*10-4 m2*K/W, has resulted in the drop of turbine set efficiency by about 0.3%. Such drop depends on a given configuration of degree of degradation of particular feed-water heaters applied in a considered turbine set.

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