Numerical study of the effect of different wavy walls on the slug heat transfer rate in microtubes using the modified fluid volume method

Author: Ahmad Hosseini

Supervisors: Dr. Reza Maddahian, Dr. Sajad Khodadadi

Abstract

The research explores how altering wall shapes and using slug two-phase flow can improve heat transfer in heated wavy wall microtubes. By employing a modified volume of fluid method, the study captures the interface between two phases and compares the heat transfer effect of a single Taylor bubble across eight wall shapes. The best shape increased heat transfer by 50% compared to smooth walls. Additionally, the study investigates the influence of wavelength and wave amplitude on heat transfer in a modified geometry of consecutive half-sines. Optimal values of 100 microns for amplitude and 20 microns for wavelength resulted in a 2.5 times increase in heat transfer compared to slug flow and an 11 times increase compared to single-phase flow in smooth walls. The combined effect of heat transfer and pressure drop was also analyzed, showing limited negative impact of increased pressure drop on heat transfer. Two accurate correlations for the average Nusselt number and TPF were presented.

1. Introduction

The innovations of the present work can be listed as follows:

The effect of various geometric functions used for the wall of a microchannel containing a slug flow to increase its thermal performance is compared, and the best geometric function is being selected based on the comparison.
The thermal performance factor for the optimally selected wavy wall is investigated to understand how pressure drop and heat transfer are combined in the selected geometry.
A relationship for predicting the average Nusselt and TPF for the optimal wall is presented.
A two-phase incompressible solver with high accuracy in capturing the interface and controlling parasitic currents to simulate slug flow in microchannels is developed.

2. Simulation Results

Fig 1: Implementation flowchart of solving equations in developed two-phase incompressible flow solver in OpenFoam open-source code.

Fig 2: Geometry and grid of the present problem

Fig 3: Schematic figures of the investigated corrugated walls

Figure 4: The schematic of the considered model and the peristaltic wave propagation

Table 1: Comparison of the Nu_max and the average of Nū of the eight proposed functions of Fig 3
Figure	3-h	3-g	3-f	3-e	3-d	3-c	3-b	3-a
"Average of Nū"	18.285	25.183	11.055	14.030	18.327	20.294	19.922	18.651
Nu_max	19.698	37.1467	16.2092	17.5016	26.0303	26.9974	28.9043	31.1529

Fig 5: Variations of the "average of " ¯("Nu" )based on half-sine wavelength at different amplitudes

3. Conclusion

The research aimed to study Taylor bubbles in wavy microtubes and find a wall geometry that enhances heat transfer. Using OpenFOAM, various geometries were tested, focusing on maintaining an acceptable pressure drop. Key findings include:

Optimal geometry for two-phase flow differs from single-phase flow.
Smooth lines and strong slopes reduce heat transfer, while rough surfaces can increase the Nusselt number by 22%.
Geometry 3-f has the lowest, and 3-g the highest Nusselt number, with averages 3 and 6.88 times higher than single-phase flow.
Higher amplitude and shorter wavelength increase surface tension and circulation, boosting heat transfer.
Larger amplitude and shorter wavelength also cause a significant pressure drop despite better heat transfer.