Experimental Research of Dynamic Instabilities in the Presence of Coiled Wire Inserts on Two-Phase Flow

Gokhan Omeroglu,Omer Comakli,Sendogan Karagoz,Bayram Sahin

doi:10.1155/2013/714180

Abstract

The aim of this study is to experimentally investigate the effect of the coiled wire insertions on dynamic instabilities and to compare the results with the smooth tube for forced convection boiling. The experiments were conducted in a circular tube, and water was used as the working fluid. Two different pitch ratios (H/D = 2.77 and 5.55) of coiled wire with circular cross-sections were utilised. The constant heat flux boundary condition was applied to the outer side of the test tube, and the constant exit restriction was used at the tube outlet. The mass flow rate changed from 110 to 20 g/s in order to obtain a detailed idea about the density wave and pressure drop oscillations, and the range of the inlet temperature was 15–35°C. The changes in pressure drop, inlet temperature, amplitude, and the period with mass flow rate are presented. For each configuration, it is seen that density wave and pressure drop oscillations occur at all inlet temperatures. Analyses show that the decrease in the mass flow rate and inlet temperature causes the amplitude and the period of the density wave and the pressure drop oscillations to decrease separately.

Highlights

In many industrial systems in which boiling heat transfer exists, the ow instabilities occurring based on the boiling heat transfer cause certain parts where heat is transferred to breakdown. e fact that the system pressure, ow rate, and similar parameters create oscillations shortens the life of the process systems
In different types of test tubes and tubes with different cross-sections, Coleman and Garimella [8] and Leung et al [9] investigated the instabilities of two-phase ow systems. e forced convection boiling in a horizontal tube was studied by Çomaklı et al [10]
In order to investigate the effects of inlet subcooling on steady and unsteady state characteristics, the experiments were conducted for three different inlet temperatures of 15∘C, 25∘C, and 35∘C and three different test tubes under constant heat input (24 kW), system pressure (7.5 bar), and exit restriction. e rst mass ow rate for each group is 110 g/s, because the ow is single phase ow at high ow rates, and the mass ow rate was decreased by 10 12 g/s slight steps to de ne the characteristic curve. e lowest ow rate was taken as 20 g/s in the experiments. ecause of the burn-out possibility, ow rates lower than 20 g/s cannot be achieved

Summary

Introduction

In many industrial systems in which boiling heat transfer exists, the ow instabilities occurring based on the boiling heat transfer cause certain parts where heat is transferred to breakdown. e fact that the system pressure, ow rate, and similar parameters create oscillations shortens the life of the process systems. The periods, and the amplitudes of the pressure drop and density wave type oscillations decreased with the decreasing mass ow rate and increased with the decreasing inlet temperature.

Results

Conclusion