INTRODUCTION

INDUSTRIAL CONTEXT

In many branches of industry involving the gasification of liquefied gases or refrigeration engineering, the operating conditions of evaporative heat exchangers are characterised by low temperature heads and small heat fluxes. Due to environmental concerns as ozone depletion and global warming, many companies have replaced the applications using freons (chlorfluorocarbons CFC and hydrochlorfluorocarbons HCFC) by hydrocarbon boiling heat transfer.

Pool boiling of propane is generally used to refrigerate installations of liquefaction of natural gas, where boiling occurs from a heated surface submerged in a large volume of stagnant liquid.

The development of enhanced surface makes possible the enhancement of heat transfer in boiling, and a decreasing of the temperature head with lower energy consumption. These technologies increase the capacity of heat transfer and solve the space limitations for existing plants, providing compact and efficient solutions.

Technip has successfully worked, together with other companies, in developing and introducing enhanced heat transfer technologies using enhanced tubes, which are fast becoming standard solutions for certain key heat exchangers in liquefied natural gas (LNG) plants and ethylene plants. The enhanced heat transfer technology performance, based on experienced thermal design and equipment engineering, is already confirmed with field data.

Due to the complexity, it not possible to take into account all the details of this enhanced structures in computational fluid dynamics (CFD) tools, so a numerical model should be developed to reproduce the real evaporation. As a first approximation, a smooth surface is frequently considered.

OBJECTIVES

The main objective of the BEI, proposed by Technip, is to numerically reproduce in NEPTUNE_CFD the boiling of propane around an immerged tube in this static liquid with an imposed heat flux.

First of all, the numerical simulations of natural convection were validated by comparing the simulation results with correlations as well as experimental data, both obtained from the literature. Two different cases were tested, imposing a constant heat flux, and imposing a constant temperature at the tube wall. Then, the two-phase flow simulation was implemented, considering the appropriate regime.

In order to validate this numerical model, CFD results were compared with experimental results obtained from literature. Once validated, different rugosity could be considered in future work, to compare with experimental data of enhanced tube, provided by Technip.

SYSTEM DESCRIPTION

The system under study is represented on the right, in Figure 1. A heated tube is immersed in stagnant high quality propane, with the following properties:

               

Figure 1 : Studied system: Pool boiling of propane around a heated tube​

The tube, with 19.05mm diameter, is considered to have a smooth surface and negligible thickness.

Different constant heat flux densities are imposed on the tube surface, varying from 2 kW/m² to 30 kW/m².