Modeling, metrological characterization and active control of evaporation processes in microchannel systems
Dissipation of high heat fluxes using microchannel cooling has many applications for thermal management in electronic cooling and process technology. Both single phase cooling and two phase cooling have been studied by different researchers in microchannels. Due to high surface to volume ratios in microchannels, microchannels are more efficient compared to traditional cooling devices. Considering latent heat transfer coefficient during phase transition, two phase cooling can provide more thermal management system. However, two phase boiling flow can make several complexities and instabilities such as pressure oscillations, dry out and back flow. Due to these instabilities, we intend to develop local sensors that read out information which is based on flow electrical properties. The main influencing properties that play a rule here are dielectric constant and conductivity.
A precise knowledge about the micro-evaporation process can help to optimize the microstructure geometry for various application scenarios. In order to gain better understanding of the physical mechanism behind boiling in a microchannel, the phase change in a single microchannel and subsequently flow interaction in two connected microchannels during evaporation process were investigated.
Experimental setup development
The experimental part of the project has the objective of implementing impedance sensors over boiling flow and using videography as a reference for sensing validation. In order to perform these measurements indium tin oxide elements are used as sensors, which is a transparent conductive material. Impedance of the two phase flow inside the channel is measured with a bench top LCR meter. After post processing of the impedance measurements, flow pattern of the two phase flow is detectable. The flow regime detection is validated with recorded videos as well. Another objective in these experiments is making a full flow regime map and data reduction of water boiling a single metallic microchannel. Two test facilities are provided both in IMTEK and KIT-IMVT to perform the parallel videography and electrical measurements on two phase boiling flow.
Fig. 1: a. fabrication process of glass lid, b. assembly of glass lid with sensors on the channel, c. photograph of fabricated glass lid, d. location of the sensors and access pads on glass lid.
|Fig. 2: Technical development of the experimental method and test setup.|
|Fig. 3 & 4: Structure of the realized micro evaporation system: 3D-CAD-drawing (left) and photography (right).|
Results and analysis
An experimental based on impedance sensing was developed for flow boiling pattern recognition in this research. Considering the visual flow regime observations and processing of the measurement results, it appears that a probability density function (PDF) analysis applied to electrical impedance data is useful to detect vapor flow regime in boiling microchannel flow. Both annular and bubbly flow have a single peak in the PDF distribution, while slug has two peaks corresponding, with magnitude relative to the average relative lengths of vapor and liquid segments in between slugs. Of these two impedance peaks seen in the slug flow PDF, it was observed that the larger one matched the magnitude of that seen in annular flow, indicating as expected the significant vapor phase void fraction within the measurement region.
|Fig. 5: Flow regime detection using probability density analysis of the impedance measurements in constant heat condition (30 Watt).|
|Fig. 6: Video frames showing flow patterns under different flow rates.||Fig. 7: Boiling water in a single metallic microchannel (1,5 mlmin - 25 Watt).|
Simulation and theoretical analysis
|Fig. 8: 2D simulation of single elongated bubble dynamic during flow boiling in a single partially heated microchannel.|
|Fig. 9: Validation of the simulation results regarding (a) bubble shape, (b) local heat transfer coefficient and (c) position of the bubble. XR: bubble back, XN: bubble front.|
We carried out the two-phase VOF simulations using interThermalPhaseChange solver in OpenFOAM. Parametric study of the predefined bubble location were performed in single and dual-channel setup with analysis of pressure, flow fields and heat transfer resulting in force balances and detailed bubble growth.
|Fig. 10: Vapor bubble growth during flow boiling in a single partially heated microchannel.|
Fig. 11: Spatio-temporal evolution of phase fraction at U = 0:05m = s in three channel directions with initial vapor bubble placement at xinit = 0.60mm.
Furthermore, we have developed the mathematical models by using a pragmatic mix of empirical correlations, numerical discretization and analytical models to cover the different flow regimes including single bubble growth, slug/elongated bubbles flow and annular flow in the evaporation process through a single microchannel. These modeling codes provide the local heat transfer coefficient and pressure drop in length of microchannel. Finally, we compared the developed flow regime based models with the obtained experimental database.