A state-of-the-art overview of recent work on boiling and two-phase flows in microchannels are reviewed, focusing primarily on the high points of recent developments. The topics covered include critical heat fluxes and their prediction in microchannels, new two-phase flow pattern maps, flow regimes and visualization in microchannels, boiling phenomena and flow instabilities in multi-microchannel cooling elements, flow boiling prediction methods specifically for microchannels and their comparison to data, trends in flow boiling data in microchannels and mechanisms that may be responsible for the seemingly divergent trends, and two-phase pressure drops in microchannels.

State-of-the-Art Overview of Boiling and Two-Phase Flows in Microchannels

https://ir.nctu.edu.tw:443/bitstream/11536/12055/1/000238269800007.pdf

Saturated flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

https://hal.archives-ouvertes.fr/hal-00540589/document

Hydrodynamics and heat transfer during flow boiling instabilities in a single microchannel

Forced convection boiling in microchannels is studied experimentally under the uniform heat flux boundary condition. Several microchannel heat sinks with integrated temperature sensors, spanning two orders of magnitude in height 5–500 µm, have been fabricated with designed nucleation sites on the bottom surfaces. The microchannels are capped by a glass wafer to monitor bubble activity using video microscopy. Distributed micro heater elements on the device backside are used as the heat source, while the working liquid flow rate is adjusted using a syringe pump. The boiling curves of the device temperature as a function of the input power have been measured for various flow rates. The curves for increasing and decreasing heat flux exhibit a hysteresis loop, while the conditions corresponding to the onset of nucleate boiling and critical heat flux (CHF) are clearly distinguishable. The activity of nucleation sites as well as the ensuing bubble dynamics, from incipience to departure, is found to depend on the channel height. The critical size above which a nucleation site is active, along with three aspects of bubble dynamics, namely growth rate, departure size and release frequency, have been characterized experimentally and proper control parameters have been identified. (Some figures in this article are in colour only in the electronic version)

/pdf/height-effect-on-nucleation-site-activity-and-size-dependent-1z2it7f5dl.pdf

Height effect on nucleation-site activity and size-dependent bubble dynamics in microchannel convective boiling

Experimental evaluation of flow boiling incipience of subcooled fluid in a narrow channel

Boiling heat transfer in vertical minichannels. Liquid crystal experiments and numerical investigations

Purpose – The purpose of this paper is to focus on the application of the Trefftz method to the calculation of the two-dimensional (2D) temperature field in the boiling refrigerant flow through an asymmetrically heated vertical minichannel with a rectangular cross-section. The considerations were limited to determining the temperature of the continuous phase – liquid for bubbly and bubbly-slug flow. The numerical solution found with the Trefftz methods was compared with the simplified solution. For nucleate boiling, heat transfer coefficient at the heating foil – liquid contact was determined. Design/methodology/approach – The Trefftz method was used to determine 2D temperature distributions for the glass pane, the heating foil and the boiling liquid. The temperature fields were approximated by the sum of the particular solution and the linear combination of suitable Trefftz functions. Coefficients of linear combination were computed using experimental data, including heating foil temperature measurements...

Application of adjustment calculus to the Trefftz method for calculating temperature field of the boiling liquid flowing in a minichannel

The paper focuses on the numerical solution to two-dimensional temperature field of boiling liquid flowing along a vertical, asymmetrically heated minichannel with a rectangular cross-section. One of the walls of a minichannel is DC supplied single-sided enhanced foil with mini-recesses distributed unevenly in the selected area. The parallel walls are made of glass panes for thermal insulation and they are intended for observation of the two-phase flow and the void fraction. The thin layer of thermosensitive liquid crystal paint on the outer side of the foil enabled to record two-dimensional temperature distribution of outer foil surface. The paper described computations based on Trefftz method for finding two-dimensional temperature field of boiling liquid flowing along the minichannel. The presented research is limited only to the liquid phase of the two-phase mixture observed in the minichannel. The velocity of liquid flowing through the minichannel is represented by a piecewise linear approximating function. To solve energy equation for liquid phase, Trefftz functions specially generated for this purpose were employed. Temperature field in the fluid was approximated by a linear combination of Trefftz functions. Equalizing calculus was applied to the Trefftz method to smooth temperature measurements and reduce measurement errors. Temperature at the interface between working fluid and foil amounts to the saturation temperature. Temperature distribution in the foil and the glass pane was also computed using proper Trefftz functions.

/pdf/equalizing-calculus-in-trefftz-method-for-solving-two-59bhcyd8uk.pdf

Sylwia Hożejowska

Papers

Experimental evaluation of flow boiling incipience of subcooled fluid in a narrow channel

Boiling heat transfer in vertical minichannels. Liquid crystal experiments and numerical investigations

Application of adjustment calculus to the Trefftz method for calculating temperature field of the boiling liquid flowing in a minichannel

Equalizing calculus in Trefftz method for solving two-dimensional temperature field of FC-72 flowing along the minichannel

Experimental investigations and numerical modeling of 2D temperature fields in flow boiling in minichannels