Dryout During Flow Boiling in a Single Circular Minichannel: Experimentation and Modelling

2008 ◽  
Author(s):  
Davide Del Col ◽  
Alberto Cavallini ◽  
Stefano Bortolin ◽  
Marko Matkovic ◽  
Luisa Rossetto
Keyword(s):  
Heat Flux ◽  
Standard Deviation ◽  
Critical Heat Flux ◽  
Wall Temperature ◽  
Flow Boiling ◽  
Temperature Fluctuations ◽  
Test Channel ◽  
Average Value ◽  
Drying Up ◽  
Experimental Values

This paper presents an experimental investigation on the dryout during flow boiling of R134a and R32 inside a 0.96 mm diameter single circular minichannel. In the present tests, the test channel is not electrically heated; instead, the flow boiling is achieved by means of a secondary fluid (water). Therefore, the heat flux is not uniform in the channel since the temperature of the water varies. The onset of dryout is detected by means of the standard deviation of the temperature readings in the wall. The wall temperature in fact displays larger fluctuations in the zone where dryout occurs, which are related to the presence of a liquid film drying up at the wall with some kind of an oscillating process. These temperature fluctuations are detected by means of the standard deviation in the wall temperature. These temperature fluctuations never appear during condensation tests, neither are present during flow boiling at low vapor qualities. The fluctuations also disappear in the postdryout zone. Experimental values of dryout quality measured with the above method are reported in this paper at mass velocity ranging between 100 and 700 kg m−2s−1 for R134a and between 200 and 900 kg m−2s−1 for R32. Since the heat flux is not uniform along the channel, each dryout point is characterized by its own boiling story. Nevertheless, an average value of heat flux can be defined in the channel, with the purpose of comparing it to critical heat flux values in uniformly heated channels. Present experimental data has been compared against some models available in the literature, which provide either the critical heat flux or the dryout quality in microchannels.

Prediction of critical heat flux in flow boiling process under the effect of different operating parameters

2020 ◽  
pp. 095765092096223
Author(s):  
Ehsan Abedini ◽  
Mohammad Behboudi ◽  
Arash Mohammadi Karachi ◽  
Reza Hamidi Jahromi ◽  
Kianoush DolatiAsl
Keyword(s):  
Heat Flux ◽  
Surface Roughness ◽  
Critical Heat Flux ◽  
Wall Temperature ◽  
Flow Boiling ◽  
Turbulence Models ◽  
Fluid Flows ◽  
Channel Length ◽  
Heated Wall ◽  
Boiling Process

In the present study numerical simulation of flow boiling process has been conducted for evaluation of critical heat flux conditions under the effect of different parameters (mass flux, heat flux, channel length and surface roughness). Comparison between the results of the present study and previous researches were done. The comparison shows a good agreement between the present study and previous researches. The three different turbulence models (k-epsilon, k-omega and Reynolds Stress) are considered for simulation of boiling heat transfer and CHF phenomenon. The highest accuracy of simulation is obtained by k-epsilon model. The results express that the wall temperature value of tube with adiabatic and heated boundary conditions for first and second half of the tube is lower than the wall temperature when the fluid flows only in the heated wall section. Reduction of velocity value also leads to reduction of maximum wall temperature value and CHF value. Decreasing Roughness value as an effective parameter leads to an increase in wall temperature. Maximum value of the wall temperature, after CHF point, also increases with increase in heat flux value. CHF depends on the surface roughness and rises with increasing roughness value.

Experimental study on critical heat flux during flow boiling of R134a in a vertical helically coiled tube

2020 ◽  
Vol 234 (15) ◽  
pp. 3094-3101
Author(s):  
Xiaojuan Niu ◽  
Huaijie Yuan ◽  
Liang Zhao
Keyword(s):  
Experimental Study ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Wall Temperature ◽  
Mass Flux ◽  
Flow Boiling ◽  
Vapor Quality ◽  
Coiled Tube ◽  
Helically Coiled Tube ◽  
System Pressure

This paper carried out an experimental study on the critical heat flux during flow boiling of R134a in a vertical helically coiled tube. The length, inner diameter, coil diameter, and pitch of the test tube were 1.85 m, 8 mm, 205 mm, and 25 mm, respectively. Experiments cover the mass flux range of 190–400 kg·m−2·s−1, heat flux of 15–55 kW·m−2, inlet pressure of 0.8–1.1 MPa, and inlet vapor quality of 0.01–0.35. The effects of critical heat flux identification method, mass flux, system pressure, and inlet vapor quality on critical heat flux were presented. The critical heat flux obtained by the wall temperature rise method was larger than that obtained by the wall temperature oscillation method. The deviation of the critical heat flux corresponding to two methods, including wall temperature rises sharply above 10 ℃ and wall temperature drastic oscillation, was about 20% under the present experimental conditions. The critical heat flux increased with mass flux while it decreased with the inlet vapor quality and pressure. The experiment data were compared with four existing empirical correlations. A new correlation is proposed for critical heat flux prediction in vertical helical tubes.

Optimized Expanding Microchannel Geometry for Flow Boiling

2013 ◽  
Vol 135 (4) ◽  
Author(s):  
Mark J. Miner ◽  
Patrick E. Phelan ◽  
Brent A. Odom ◽  
Carlos A. Ortiz ◽  
Ravi S. Prasher ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Boiling ◽  
Flow Direction ◽  
Convective Heat Transfer Coefficient ◽  
Experimental Studies ◽  
Separated Flow ◽  
Channel Geometry ◽  
Experimental Values

This study discusses the simulation of flow boiling in a microchannel and numerically predicts the effects of channel geometry variation along the flow direction. Experimental studies by Pan and collaborators and suggestions from Mukherjee and Kandlikar have generated interest in expanding the cross section of a microchannel to improve boiling heat transfer. The motivation for this geometry change is discussed, constraints and model selection are reviewed, and Revellin and Thome's critical heat flux criterion is used to bound the simulation, via matlab, of separated flow in a heated channel. The multiphase convective heat-transfer coefficient is extracted from these results using Qu and Mudawar's relationship and is compared to reported experimental values. Expanding channel geometry permits higher heat rates before reaching critical heat flux.

scholarly journals Predicting Critical Heat Flux With Multiphase CFD: 4 Years in the Making

2017 ◽  
Author(s):  
Emilio Baglietto ◽  
Etienne Demarly ◽  
Ravikishore Kommajosyula
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Heated Surface ◽  
Force Balance ◽  
Modeling Framework ◽  
Lift Off ◽  
Limiting Condition

Advancement in the experimental techniques have brought new insights into the microscale boiling phenomena, and provide the base for a new physical interpretation of flow boiling heat transfer. A new modeling framework in Computational Fluid Dynamics has been assembled at MIT, and aims at introducing all necessary mechanisms, and explicitly tracks: (1) the size and dynamics of the bubbles on the surface; (2) the amount of microlayer and dry area under each bubble; (3) the amount of surface area influenced by sliding bubbles; (4) the quenching of the boiling surface following a bubble departure and (5) the statistical bubble interaction on the surface. The preliminary assessment of the new framework is used to further extend the portability of the model through an improved formulation of the force balance models for bubble departure and lift-off. Starting from this improved representation at the wall, the work concentrates on the bubble dynamics and dry spot quantification on the heated surface, which governs the Critical Heat Flux (CHF) limit. A new proposition is brought forward, where Critical Heat Flux is a natural limiting condition for the heat flux partitioning on the boiling surface. The first principle based CHF is qualitatively demonstrated, and has the potential to deliver a radically new simulation technique to support the design of advanced heat transfer systems.

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