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

doi:10.1016/S0894-1777(01)00113-3

Experimental Thermal and Fluid Science

Volume 25, Issue 7, January 2002, Pages 547-555

https://doi.org/10.1016/S0894-1777(01)00113-3 Get rights and content

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.

Introduction

In the frame of the JOULE-programme of the European Commission [1] enhanced heat transfer surfaces are tested. The background of the experimental investigations is to improve the heat transfer, especially for low and medium heat fluxes (< $50 kW / m^{2}$ ), as they are encountered in many present and especially future applications of industrial compact heat exchangers (e.g., process industry). The advantages of compact heat exchangers compared with conventional heat exchangers are their higher efficiency, their smaller volume and the smaller wall superheat. This leads to savings of raw materials and energy, e.g., during the manufacture of such heat exchangers. The improved safety should also be mentioned, e.g., when dangerous fluids are used (smaller fluid inventory).

The conventional way to describe the heat transfer characteristics of a heat transfer surface is the determination of the heat transfer coefficient α. Dependent on the boundary conditions, the α values are determined. Based on measurements of heat input, saturation temperature of the working fluid and surface temperature, empirical correlations for α are generated that include various dimensionless groups comprising thermophysical properties of the fluid and geometric parameters of the surface.

Beside the experimental investigations of α, numerical simulations of the basic evaporation process itself become more and more important. In this context, boiling parameters like the bubble growth, the bubble departure diameter and the bubble generation frequency are of interest because they can be used for verification of the numerical models. Therefore, visualization experiments of the boiling phenomena are needed to extract these variables from digitized images. As a rule, the fast evaporation processes require high-speed video systems for recording, and consequently due to the great amount of image data, digital image processing techniques have to be employed for quantitative data evaluation.

Section snippets

Experimental set-up

Fig. 1 shows a schematic arrangement of the experimental apparatus. The test vessel is designed for a maximum pressure of 16 bar. Two cryostats are connected to the vessel cooling down the liquid as well as the vapour. The fluid tank with a finned tube heat exchanger stores the fluid during a change of the test specimen.

The test vessel and the condenser are instrumented with thermocouples (NiCr–Ni, type K), to measure the liquid and vapour temperatures and with pressure sensors to measure the

Experimental results

For the heat transfer surface GEWA-PB [3], made of the wall material St35.8 with the structured shape shown in Fig. 3, visualization experiments were carried out for low heat fluxes ( $q<10 kW / m^{2}$ ) whereas the heat transfer coefficients were determined for heat fluxes up to $100 kW / m^{2}$ .

Conclusions

High-speed flow visualization of pool boiling (working fluid propane) from structured tubular heat transfer surfaces is carried out. By means of digital image processing, qualitative and quantitative evaluations are performed. The boiling parameters, bubble departure diameter, bubble generation frequency and bubble flow velocity, are quantitatively determined for two low heat fluxes (q=2.2 and $5 kW / m^{2}$ ). The visualization provides both a better insight into the thermofluid dynamic phenomena

Acknowledgements

This work has been partly funded by the European Commission within the frame of the JOULE programme (project no. JOE3-CT97-0061).

References (4)

R. Mertz et al.
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A method of tracking ensembles of particle images
Experiments in Fluids
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