Heat transfer of bubbly flow on inner wall of annular channel

Experimental investigations of heat transfer from the heated wall to the two-phase bubbly flow were performed in vertical annular channel using air-water system. The IR-thermography and miniature temperature sensors were used to measure heat transfer coefficients. The influence of bubbles on heat transfer is shown in comparison with the case of single phase flow. The presence of bubbles in the flow leads to heat transfer intensification in the annular channel even for low void fractions.


Introduction
An important problem of creating the energy-efficient equipment is the study of passive heat transfer intensifiers.One of the simplest examples of such equipment is a pipe-in-pipe heat exchanger in which heat is transferred between the flows in the inner and outer pipes and the medium around the outer pipe.
Often the temperature of the medium in such heat exchangers exceeds the critical one that causes boiling of the coolant.This leads to the appearance of vapor bubbles in the flow.Despite a rather large number of studies, a number of questions are still open even for a single-phase flow.Studies of the local structure of the two-phase gas-liquid flow, and especially heat transfer in such flows, are very limited in the literature [1][2][3][4][5][6].
Therefore, studying the local structure of bubble gas-liquid flow in annular channels as well as the effects of gas bubble addition on heat transfer is a relevant task.

Experimental setup
The experimental setup was a closed circuit in a liquid and an open circuit in the gas phase.The test liquid, i.e. distilled water, was pumped by the centrifugal pump to the test section of the main tank.The temperature of the liquid was controlled at the level of 25 ± 0.2 ° C. The flow rate was monitored using an ultrasonic flowmeter.
The section, simulating the pipe-in-pipe heat exchanger, was an axisymmetric annular channel (Fig. 1  The gas flow rate was determined using Bronkhorst (up to 1 l/min) and Aalborg (up to 5 l/min) gas flow controllers.The measurement error of liquid and gas flow rates was ±2 % of the measured value.
Generation of bubbles was carried out by feeding air into the liquid flow through the capillaries of 50 mm length and an internal diameter of 0.7 mm, uniformly distributed in the channel cross-section.Twenty four capillaries were used for the annular channel.Bubbles' size measurements were carried out using video shooting with shadow illumination and subsequent image processing.To reduce optical distortion a box with a square crosssection, filled with immersion liquid, was used.
For experimental studies of heat transfer from the heated wall to the bubble flow, we used the test section, made of thin-walled stainless steel [7] (Fig. 1 b).The inner diameter of the test section was 42 mm, the wall thickness was 0.5 mm, and the heating area length was 500 mm.The section was heated using electric current.Current was supplied using a laboratory power source with power up to 2 kW, used for power control.

Conclusions
The study of heat transfer the heated wall to single-and two-phase flows has been carried out.It is shown that the addition of gas bubbles to the flow leads to the heat transfer intensification.At that, the significance of this effect decreases with increasing Reynolds number of liquid that was reported earlier by other authors.The effects of heat transfer increase due to appearing bubbles should be taken into account in the calculations when adding a dispersed phase in the flow or in the beginning of boiling.
We can conclude that at small Reynolds numbers in the liquid phase, the addition of the dispersed phase is an effective method of heat transfer intensification.However, with increasing Re the effect of bubbles on heat transfer decreases.
a).To center the inner tube we used the distancing elements installed to avoid flow disturbance in the measurement area.The hydraulic diameter of the annular channel (Dh= D -d) was equal to 22 mm.The length of the flow stabilization area before the measurement region was 80 Dh.

This
work was supported by the Russian Foundation for Basic Research (project No. 15-38-21040).