Research project

Experimental Investigation of Microscale Flow Boiling Using Interferometry and Temperature-Sensitive Paint

Microscale flow boiling is a promising concept for compact high-performance cooling systems, as it can provide very high heat transfer coefficients within small channel geometries. However, reliable prediction of heat transfer and critical heat flux in microchannels remains an active field of research. Existing correlations often show large deviations outside their original experimental range, mainly because the local interaction between bubble dynamics, flow pattern, surface temperature and microlayer evaporation is not yet fully understood. [1][2][3]

This research project aims to combine two high-resolution optical measurement techniques to investigate flow boiling heat transfer dynamics. Namely Temperature-Sensitive Paint (TSP) and LED Interferometry:

Temperature-Sensitive Paint

Temperature-sensitive paint (TSP) enables optical measurement of surface temperature fields. A temperature dependent fluorescent paint layer is applied on top of an optical transparent heating wall (Indium Tin Oxide (ITO) on a sapphire substrate). The TSP can be excited by an UV-LED light. The fluorescent signal intensity depends on the surface temperature of the TSP. That way the temperature field between heating wall and cooling fluid can be mapped, visualizing individual bubbles, dry patches, rewetting areas and flow patterns. From the temperature fields heat flux and heat transfer coefficient can be derived. [4][5]

LED Interferometry

During bubble growth, a thin liquid microlayer can form between the vapor bubble and the heated wall. The microlayer thickness can reach up to 5µm. Microlayer and triple contact line evaporation strongly contribute to the overall heat transfer. LED-based interferometry uses light reflected at the microlayer interfaces (wall-liquid and liquid-vapor) to generate interference fringes. The thickness and geometry of the microlayer and the occurrence of dry spots in the middle of the bubble can be measured by this technique. [6]

Measurement Concept

The project aims to combine TSP and LED interferometry in one experimental setup. A transparent sapphire substrate with an ITO heating layer will provide optical access and controlled heating. TSP will measure the transient wall temperature distribution, while LED interferometry will resolve the microlayer beneath growing vapor bubbles. A high-speed camera is supposed to capture both signals in an alternating measurement sequence, enabling almost synchronous measurements of wall temperature, local heat flux, heat transfer coefficient, microlayer thickness, dry area, bubble growth and flow pattern. The research idea is driven by the synergy of the two measurement approaches: both measurements use the ITO heater on sapphire substrate as the optical window inside the flow, both can be measured by the same high-speed camera, and both signals can be generated by an LED-Illuminator.

References

[1] Sung-Min Kim, Issam Mudawar: “Review of databases and predictive methods for heat transfer in condensing and boiling mini/micro-channel flows”

[2] Lixin Cheng, Guodong Xia: “Fundamental issues, mechanisms and models of flow boiling heat transfer in microscale channels”

[3] Gherhardt Ribatski, Leszek Wojtan, John R. Thome: “An analysis of experimental data and prediction methods for two-phase frictional pressure drop and flow boiling heat transfer in micro-scale channels”

[4] Soumei Baba, Shimpei Saito, Naoki Takada, Satoshi Someya: “Visualization of flow boiling heat transfer using temperature sensitive paint with high spatial and temporal resolution”

[5] Zhongqi Liu, Yan Yan, Wenxuan Pu, Xiaolong Ma, Dongsheng Wen: “High spatiotemporal resolution surface temperature measurement using temperature-sensitive paints for phase change studies”

[6] A. Kossolapov, B. Phillips, M. Bucci: “Can LED lights replace lasers for detailed investigations of boiling phenomena?”