Rainer Mertz

High speed flow visualization of pool boiling from structured tubular heat transfer surfaces

Abstract Experimental investigations of pool boiling from novel tubular heat transfer surfaces (structured tubes with re-entrant cavities) are carried out with the hydrocarbon propane as working fluid operating under moderate heat fluxes ( 100 W / m 2 ) and saturation conditions. The heat transfer coefficients are presented. The evaporation phenomena are visualized by a high-speed video system and by means of digital image processing techniques (e.g., Fourier-analyses, correlation-techniques). The bubble departure diameter at the surface, the bubble generation frequency at corresponding nucleation sites and the bubble upward flow velocity are quantitatively determined.

Bubble dynamics of boiling of propane and iso-butane on smooth and enhanced tubes

Abstract Visualization experiments were carried out for nucleate pool boiling of propane and iso-butane on a horizontal smooth tube and two kinds of structured enhanced tubes. Some bubble dynamic parameters were measured. Comparisons were made between the smooth tube and the enhanced tubes for bubble growth rate, departure diameter, frequency, active site density, rise velocity and latent heat transfer. The effects of physical properties on the bubble dynamics are discussed. The present experimental results are compared with the model of Haider and Webb, relatively good agreement is found for enhanced surfaces. The augmentation mechanisms of heat transfer from structured enhanced tubes are also discussed.

Experimental study on macroscopic contact line behaviors during bubble formation on submerged orifice and comparison with numerical simulations

The formation of bubbles on an orifice plate involves a moving contact line, especially in case of poor wetting conditions. The dynamics of the moving contact line and contact angle have a significant impact on the bubble departure size. This work presents visualization and numerical simulations on gas bubble formation on submerged orifices of different wettabilities and diameters. The transient bubble shape (image), departure diameter, contact line diameter, apparent contact angle, neck diameter, and neck height are obtained under various conditions. Under certain wetting and flow conditions, the relationship between moving contact line and apparent contact angle can be relatively well defined. However, this relationship cannot be generalized for other conditions. Two kinds of contact line models have been re-evaluated against experiments, one is a contact line velocity dependent model (Model-A) and the other is a stick-slip model (Model-B). Numerical simulations were performed using Model-B and the resu...

Minichannel Heat Transfer: An Overview on Activities in Europe

An overview will be given about investigations on heat and mass transfer in narrow channels and narrow cavities, from work carried out in the last years up to the current status of research of some relevant scientific groups in Europe. The major topics of this report are evaporation heat transfer and the flow boiling pressure drop in narrow channels; microscale heat and mass transfer phenomena in pool boiling from enhanced evaporator tubes with sub-surface channels are also addressed. In the last years a challenging topic has been the enhancement of the efficiency of heat exchangers by employing micro-structured heat transfer surfaces. The need for smaller heat exchangers with higher heat transfer rates and/or smaller thermal approaches is caused by the ongoing miniaturisation of mechanical and electronic components, leading to higher heat fluxes which can damage or even destroy the components. On the other hand, enhanced heat transfer in big equipment, e.g. heat exchangers for the petrochemical and chemical industries, can lead to significant materials and energy savings and thus reduce environmental pollution. Therefore the European Union, European industries and national organisations have supported various projects to develop and to investigate a new generation of heat transfer surfaces, to better understand the related heat transfer phenomena and to model the heat transfer from these micro heat exchanger elements. There is a very extensive research in this scientific field, comprising both flow boiling and pool boiling. The present paper deals with heat transfer in narrow channels and/or cavities and with the flow boiling pressure drop occurring during heat and mass transfer in narrow channels. Investigations of major European institutions, carried out in the past and at the moment will be presented as a contribution to the overview on the current state-of-the-art in Europe, without claim of completeness. Some recent results on microscale pool boiling and flow boiling obtained in our institute will also be presented (Shuai et al., 2002; Kulenovic et al., 2002; Chen et al., 2002a, b).Copyright

Improved evaporation heat transfer surfaces for cost-effective compact heat exchangers for the process industries

Various aspects of intensified evaporation heat transfer from enhanced surfaces were investigated, both experimentally and employing computational fluid dynamics. The pool boiling performances of enhanced tubular and plate heat exchanger surfaces were tested with various process fluids. In addition, investigations of the local heat transfer on the gas side of heat exchanger elements were carried out.

Steady-State and Transient Performance of a Miniature Loop Heat Pipe

A series of tests have been carried out with a miniature loop heat pipe (mLHP), which has been developed for consumer electronics cooling, for horizontal and four vertical orientations under different sink temperatures. The mLHP has a cylindrical evaporator of 5 mm outer diameter and 29 mm length. The steady-state operating characteristics are similar for different orientations except for the orientation where the evaporator is above the compensation chamber. At an evaporator temperature of 75 °C, an evaporator heat load up to 70 W can be reached with thermal resistance of about 0.2 °C/W. The transient behavior of the mLHP is studied in detail. In general, the mLHP can be started up with very low power input (5 W). Big temperature oscillations in the liquid line were found in many cases, however, the temperature oscillations in the evaporator are minimum. The orientations greatly influence the operating characteristics of the mLHP. At least for the horizontal orientation, the overall performance of the tested mLHP is satisfying.Copyright

Visualization and Flow Pattern Map for Flow Boiling of Water in a Vertical Micro Channel

Experiments on flow boiling of water are carried out for a rectangular vertical channel with dimension (width × depth) 0.86 × 2.0 mm2 (hydraulic diameter 1.2 mm). The Confinement number, Co = [σ/(g(ρL − ρG )dh 2 )]0.5 , for the investigated range of operating parameters is about 2.2. So, applying the criterion Co > 0.5 for micro channels, the investigated channel has to be classified as “micro”. Water as working fluid is used with different mass and heat fluxes. Investigations of heat transfer and pressure drop were carried out. Results of flow visualisation are presented. On the basis of observations, a flow pattern map is obtained and the results are compared with existing flow pattern transition boundaries proposed by Taitel et al. (1980) and Mishima and Ishii (1984). The paper contains recent experimental results of an ongoing EU-project. Previous results have been presented in Shuai et al. (2002, 2003).Copyright

Numerical Study on the Formation of Taylor Bubbles in Capillary Tubes

Numerical simulations have been carried out for the transient formation of Taylor bubbles in a nozzle/tube co-flow arrangement by solving the unsteady, incompressible Navier-Stokes equations. A level set method was used to track the two-phase interface. The calculated bubble size, shape, liquid film thickness, bubble length, drift velocity, pressure drop and flow fields of Taylor flow agree well with the literature data. For a given nozzle/tube configuration, the Taylor bubble formation is found to be mainly dependent on the relative magnitude of gas and liquid superficial velocity. However, even under the same liquid and gas superficial velocities, the change of nozzle geometry alone can lead to a big change in the size of Taylor bubbles and the pressure drop behavior inside a given capillary. This indicates that the widely used flow pattern map presented in terms of liquid and gas superficial velocities is not unique.Copyright

Heat Transfer Mechanism and Flow Pattern During Flow Boiling of Water in a Vertical Narrow Channel: Experimental Results

Experimental investigations on flow boiling phenomena in a vertical narrow rectangular microchannel with the hydraulic diameter dh = 0.48 mm were carried out. The experiments were performed under fluid-inlet subcooling conditions with deionised and degassed water for different mass fluxes. Investigations on pressure drop and heat transfer during single-and two-phase flow have been carried out. Moreover, flow visualisation of the two-phase flow patterns along the channel was performed using a digital high-speed video camera. The present work outlines local heat transfer coefficients for three mass fluxes (200, 700 and 1500 kg/m2 s) and heat fluxes (30–110, 35–150 and 65–200 kW/m2 , respectively) during two-phase flow. The fluid temperature at the inlet was about 50 °C what corresponds to inlet subcooling, depending on flow pressure conditions, from 34 °C to 57 °C. The visual observations were used to obtain a better insight about the heat transfer mechanism.© 2006 ASME

Experimental Investigation of Flow Boiling in a Vertical Narrow Channel

Experimental investigations on thermofluid-dynamic phenomena in a vertical narrow rectangular microchannel with the hydraulic diameter dh = 0.27 mm were carried out. The experiments are performed under fluid-inlet subcooling conditions with de-ionised and degassed water for different mass fluxes (50–2000 kg/m2 s) and heat fluxes (2–150 kW/m2 ). Moreover, flow visualisation of the two-phase flow patterns along the channel is performed using a digital high-speed video camera. Investigations on pressure drop during single- and two-phase flow have been carried out. The present work is concentrated on two-phase heat transfer. The mean heat transfer coefficient and the local heat transfer coefficient at saturated conditions were calculated and the latter ones was compared with available correlations.Copyright