High Resolution Measurement of Wall Temperature Distribution During Forced Convective Boiling in a Single Minichannel

Abstract

Cooling systems incorporating convective boiling in mini- and microchannels achieve very high thermal performance. Although many investigations related to the subject have already been conducted, the basic phenomena of the heat transfer mechanisms are not yet fully understood. The development of empirical correlations based only on flow pattern maps does not lead to a deeper knowledge of the mechanisms. In this study a comprehensive measurement technique that was successfully adapted in pool boiling experiments [8,9] was used for the investigation of forced convective boiling of FC-72 in a single rectangular minichannel. This technique allows the measurement of the local temperature with very high spatial and temporal resolution. High speed video recording was used to observe the flow inside the minichannel. The inlet Reynolds number was kept constant for the first measurements to Re = 200 corresponding to a hydraulic diameter of the minichannel of 800 μm. The Bond number for the proposed setup is about Bo ≈ 1.2. Several flow pattern regimes such as bubbly flow, slug flow and partially dryout were observed for heat fluxes up to 25 kW / m2. From an energy balance at each pixel element of the thermographic recordings the local transient heat flux could be calculated and compared to the flow pattern video recordings. The results of the first experiments already give an indication about the heat transfer mechanisms at different flow regimes.

A Critical Overview on the Recent Literature Concerning Flow Boiling and Two-Phase Flows Inside Micro-Scale Channels

“…Figure 20 displays the simultaneous images of the elongated bubble and the heated flux distribution. On contrary to the three-zone model, the results by Schweizer et al [108] revealed two areas of high heat transfer rates concerning the front and back regions of the elongated bubble. These authors suggested that this behavior is related to the intense evaporation in the three-phase contact line occurring in these regions.…”

“…The present author believes that secondary flows promoting micro-convection effects close to the bubble back can be related to the heat transfer enhancement close to this region. However, it is important to highlight that the wall temperature distribution visualized by Schweizer et al [108] was obtained under low heat flux and mass velocity conditions. Additional results for high heat flux and mass velocity as those obtained by Schweizer et al [108] are still necessary in order to assume that the scenario displayed in Figure 20 can be extended to such conditions.…”

“…However, it is important to highlight that the wall temperature distribution visualized by Schweizer et al [108] was obtained under low heat flux and mass velocity conditions. Additional results for high heat flux and mass velocity as those obtained by Schweizer et al [108] are still necessary in order to assume that the scenario displayed in Figure 20 can be extended to such conditions. Consolini and Thome [21] pointed out that different heat transfer coefficient behaviors may be attained when performing flow boiling experiments under stable or unstable conditions.…”

“…Downloaded by [University of Illinois Chicago] at 03:59 03 December 2014 According to the three-zone model proposed by Thome and coworkers [105], the transient evaporation of the thin liquid films surrounding elongated bubbles is the dominant heat transfer mechanism, and a heat transfer peak is present when the film thickness is minimum just before surface dryout. Recently, Schweizer et al [108] obtained simultaneous images of the elongated bubble flow and the temperature distribution of the backside of a heating foil as part of the channel walls. The flow images were obtained through the transparent wall in the topside of the channel with a high-speed camera, and the temperature distribution was obtained through infrared thermography.…”

History, Advances, and Challenges in Liquid Flow and Flow Boiling Heat Transfer in Microchannels: A Critical Review

“…Active control to provide stable operation has been recently proposed [166]; however, research on developing passive techniques would be highly desirable from operational and cost standpoints. Use of infrared imaging provides a valuable tool in evaluating the transient processes inside channels without the need for optically transparent window [182,183].…”