Convective Flow Boiling

John C. Chen

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000723854
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

This book comprises selected papers from the First International Conference on Convective Flow Boiling. The purpose of the conference is to examine state-of-science and recent developments in technology of flow boiling, i.e., boiling systems which are affected by convective flows.

SATURATED CONVECTIVE BOILING

FLOW BOILING IN HORIZONTAL AND VERTICAL TUBES:THE EFFECT OF TUBE ORIENTATION ON HEAT TRANSFER

N. Kattan, J.R. Thome and D. Favrat

Industrial Energetics Laboratory Swiss Federal Institute of Technology - Lausanne CH-1015 Lausanne, Switzerland

ABSTRACT

As part of a comprehensive study on factors affecting flow boiling in horizontal tubes, local boiling heat transfer coefficients were measured for R-134a evaporating inside the same 12.00 mm bore, plain tube for horizontal flow, vertical upflow and vertical downflow over a wide range of mass velocities, vapor qualities and heat flux for a direct comparison of the effects on tube orientation on heat transfer. The experiments show a significant effect of flow direction on local heat transfer coefficients with upflow giving the highest values, horizontal flow giving significantly lower values and downflow yielding even lower values. A flashing/subcooling effect on the heat transfer process caused by the negative/positive pressure gradient for upflow/downflow, respectively, is proposed as an important new factor affecting flow boiling.

INTRODUCTION

A comprehensive, stepwise study on factors affecting flow boiling is being undertaken in the LENI laboratory. Presently, local boiling heat transfer coefficients are reported for R-134a evaporating inside the same 12.00 mm bore, plain tube for horizontal, vertical upward and vertical downward flow orientations at nearly identical pressures, mass vélocités, vapor qualities and heat fluxes. The objective it to make a direct experimental comparison of local boiling heat transfer coefficients to determine the effects of flow direction on the intube evaporation process.

In industrial practice, flow boiling inside horizontal tubes nearly always refers to refrigerants evaporating in direct-expansion evaporators with an inlet vapor quality of 0.1–0.25 and an exit condition of a few degrees of superheat. Instead, the most widely quoted flow boiling correlations have been developed from large databanks for vertical upflow with the majority the data in the vapor quality range from 0–0.5. Some of these correlations have then been extended to evaporation inside horizontal tubes using a horizontal tube databank.

Several weak points exist in this approach. First of all, above the stratified flow threshold criterion, it is assumed that there is no tube orientation effect on heat transfer, below the threshold, the reduction in the heat transfer coefficient (because the tube circumference is only partially wetted with liquid and dry at the top) is predicted by adding an empirical correction term to the vertical tube correlation. However, these empirical corrections have been developed by statistical regression to improve the fit of the vertical tube correlation to the horizontal tube boiling databank rather than by direct comparison of experimental test data for vertical and horizontal flows at the same local test conditions. Consequently, effects other than stratification may be involved and the empirical correction to heat transfer is not based on a fundamental analysis of the flow. Hence, these are potentially important weak points in existing design correlations for horizontal tubes.

Another potential weakness is related to flow patterns. Two-phase flow patterns are known to be influenced by tube orientation since the vapor is buoyant and tends to migrate towards the top half of a horizontal or inclined tube. Hence flow boiling heat transfer coefficients can be expected to be affected by this modification of the flow pattern, even for unstratified flow regimes.

Yet another potential weakness is faced at high vapor qualities. In vertical upflow boiling, dryout or the critical heat flux is reached in the vapor quality range from 0.5 < x < 0.75, thought to occur simultaneously around the tube circumference at that height. Hence vertical tube databanks, dominated by electrically-heated tube data, tend to contain few high vapor quality data points in the range 0.5 < x < 1.0. In direct-expansion evaporators (i.e. with horizontal tubes) the heat fluxes are not large enough to reach the critical heat flux and dryout begins at the top of the tube because of a deficiency of liquid. Dryout then proceeds axially along the tube from the top towards the bottom until the liquid in the lower portion of the tube either dries out completely or becomes entrained as mist in the vapor flow [refer to Figure 1 taken from Collier and Thome (1994)]. Hence, complete dryout may not occur until the vapor quality approaches 1.0 in a horizontal tube and a significant length of the tube has a local heat transfer coefficient controlled by wet wall convective heat transfer over the bottom portion of the tube and mist flow heat transfer over the upper portion. The ability of vertical tube correlations to model heat transfer in horizontal tubes in this vapor quality region is therefore questionable.

As an earlier part of the same study, experimental data on flow patterns and the threshold between stratified and unstratified flow (or better denoted as transition from all wet wall to partially...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• KEYNOTE PAPERS
• BOILING INCEPTION AND SUBCOOLING BOILING
• SATURATED CONVECTIVE BOILING
• CRITICAL HEAT FLUX AND TRANSITION-FILM-BOILING
• MULTICOMPONENT BOILING
• SPECIAL SYSTEMS AND EFFECTS
• INDUSTRIAL APPLICATIONS AND DEVELOPMENTS
• Index