System Instability of Evaporative Micro-Channels

2009 ◽  
Author(s):  
Hee Joon Lee ◽  
Shi-Chune Yao
Keyword(s):  
Heat Flux ◽  
Liquid Phase ◽  
Mass Flux ◽  
Surface Heat Flux ◽  
Surface Heat ◽  
High Mass ◽  
Phase Flow ◽  
Two Phase ◽  
Constant Flow Rate ◽  
Micro Channels

During the operation of parallel evaporative micro-channels, system instability may occur in terms of cyclical fluctuations at a long period. This is due to the co-existence of the liquid phase flow at high mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow-rate pumping, with a pressure regulating tank, and a constant heating pre-heater, the system may experience severe alternations between these two states of boiling and non-boiling with a period of minutes. This cyclical system instability has been modeled. In the model, the existence of the liquid phase flow happens at conditions of inlet subcooling and low surface heat flux that the boiling inception is hard to occur. Accordingly, the system instability criteria are established in terms of a System binary states parameter S and a non-dimensional surface heat flux, which is normalized with the boiling incipient heat flux. This model has been validated experimentally.

scholarly journals What Determines the Distribution of Shallow Convective Mass Flux through a Cloud Base?

2017 ◽  
Vol 74 (8) ◽  
pp. 2615-2632 ◽  
Author(s):  
Mirjana Sakradzija ◽  
Cathy Hohenegger
Keyword(s):  
Heat Flux ◽  
Convective Heat ◽  
Mass Flux ◽  
Surface Heat Flux ◽  
Sensible Heat Flux ◽  
Surface Heat ◽  
Cloud Base ◽  
Thermodynamic Efficiency ◽  
Tropical Ocean ◽  
Heat Cycle

Abstract The distribution of cloud-base mass flux is studied using large-eddy simulations (LESs) of two reference cases: one representing conditions over the tropical ocean and another one representing midlatitude conditions over land. To examine what sets the difference between the two distributions, nine additional LES cases are set up as variations of the two reference cases. It is found that the total surface heat flux and its changes over the diurnal cycle do not influence the distribution shape. The latter is also not determined by the level of organization in the cloud field. It is instead determined by the ratio of the surface sensible heat flux to the latent heat flux, that is, the Bowen ratio B. This ratio sets the thermodynamic efficiency of the moist convective heat cycle, which determines the portion of the total surface heat flux that can be transformed into mechanical work of convection against mechanical dissipation. The thermodynamic moist heat cycle sets the average mass flux per cloud 〈m〉, and through 〈m〉 it also controls the shape of the distribution. An expression for 〈m〉 is derived based on the moist convective heat cycle and is evaluated against LES. This expression can be used in shallow cumulus parameterizations as a physical constraint on the mass flux distribution. The similarity between the mass flux and the cloud area distributions indicates that B also has a role in shaping the cloud area distribution, which could explain its different shapes and slopes observed in previous studies.

Three Boiling Instability Modes in Silicon Microchannels

2003 ◽  
Author(s):  
H. Y. Wu ◽  
Ping Cheng
Keyword(s):  
Heat Flux ◽  
Mass Flux ◽  
High Mass ◽  
Flow Mode ◽  
High Heat Flux ◽  
Pressure Oscillations ◽  
Two Phase ◽  
Experimental Conditions ◽  
Instability Modes ◽  
Temperature And Pressure

Depending on the heat flux, mass flux, and subcooling of inlet water, three boiling instability modes in silicon microchannels are possible. These are: the LTAF (Liquid/Two-phase Alternating Flow) mode, the CTF (Continuous Two-phase Flow) mode, and the LTVAF (Liquid/Two-phase/Vapor Alternating Flow) mode. It is found that the LTAF mode occurs at low heat flux and high mass flux, and has medium-amplitude temperature and pressure oscillations. The CTF mode appears at the medium heat flux and medium mass flux, and has small-amplitude temperature and pressure oscillations. The LTVAF mode appears at high heat flux and low mass flux, and has large-amplitude temperature and pressure oscillations. During the two-phase period of the LTAF mode, bubbly flow is found to be the dominant flow pattern. Some peculiar flow patterns are observed during the two-phase period of CTF and LTVAF modes under the experimental conditions.

Variations of Buoyancy-Induced Mass Flux From Single-Phase to Two-Phase Flow in a Vertical Porous Tube With Constant Heat Flux

1999 ◽  
Vol 121 (3) ◽  
pp. 646-652 ◽  
Author(s):  
T. S. Zhao ◽  
Q. Liao ◽  
P. Cheng
Keyword(s):  
Heat Flux ◽  
Mass Flux ◽  
Single Phase ◽  
Two Phase Flow ◽  
Constant Heat Flux ◽  
Heat Fluxes ◽  
Phase Flow ◽  
Constant Heat ◽  
Two Phase ◽  
Porous Tube

This paper presents an experimental study of a buoyancy-induced flow of water with phase-change heat transfer in a vertical porous tube heated at a constant heat flux. Experiments were carried out from subcooled liquid flow to connective boiling by varying the imposed heat fluxes. At a prescribed heat flux the steady-state mass flux of water, as well as the temperatures along the tube wall and along the centerline of the packed tube, were measured. It is shown that for both single-phase flow and the two-phase flow with a rather low vapor fraction, the induced mass flux increased as the heat flux was increased. However, as the imposed heat flux was increased further, the induced mass flux dropped drastically, and remained relatively constant afterwards. The influences of various parameters such as the porous tube diameter, the particle sizes, and the hydrostatic head on the induced mass flux are also examined.

Impact of Channel Geometry on Two-Phase Flow Heat Transfer Characteristics of Refrigerants in Microchannel Heat Exchangers

1998 ◽  
Vol 120 (2) ◽  
pp. 485-491 ◽  
Author(s):  
T. S. Ravigururajan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Mass Flux ◽  
Two Phase Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Heat ◽  
The Heat Transfer Coefficient

Microchannel surfaces, often machined to 20 to 1000 μm in width and depth, are employed in high-heat-flux applications. However, a large number of variables, control the two-phase flow heat transfer coefficient. The pressure, the surface heat flux, and the mass flux significantly affect the thermal transport. Experiments were conducted on a setup that was built for testing microchannel heat exchanges. The parameters considered in the study are power input: 20 to 300 W, volume flow rate: 35 to 300 ml/min, quality: 0 to 0.5, inlet subcooling: 5 to 15°C. The results indicate that the heat transfer coefficient and pressure drop are functions of the flow quality, the mass flux, and, of course, the heat flux and the related surface superheat. The heat transfer coefficient decreases from a value of 12,000 W/m2-K to 9000, W/m2-K at 80°C, when the wall superheat is increased from 10 to 80°C. The coefficient decreases by 30 percent when the exit vapor quality is increased from 0.01 to 0.65.

Two-Phase Flow Patterns During Microchannel Vaporization of CO2 at Near-Critical Pressures

2003 ◽  
Author(s):  
Jostein Pettersen
Keyword(s):  
Heat Transfer ◽  
Flow Pattern ◽  
Mass Flux ◽  
Two Phase Flow ◽  
Bubble Formation ◽  
Glass Tube ◽  
Flow Patterns ◽  
High Mass ◽  
Phase Flow ◽  
Two Phase

Carbon dioxide (CO2 / R-744) is receiving renewed interest as a refrigerant, in many cases based on systems with microchannel heat exchangers that have high pressure capability, efficient heat transfer, and compact design. A good understanding of two-phase flow of evaporating CO2 in microchannels is needed to analyze and predict heat transfer. A special test rig was built in order to observe two-phase flow patterns, using a horizontal quartz glass tube with ID 0.98 mm, externally coated by a transparent resistive film. Heat flux was obtained by applying DC power to the film, and flow patterns were recorded at 4000 or 8000 frames per second by a digital video camera. Flow patterns were recorded for temperatures 20°C and 0°C, and for mass flux ranging from 100 to 580 kgm−2s−1. The observations showed a dominance of intermittent (slug) flow at low x, and wavy annular flow with entrainment of droplets at higher x. At high mass flux, the annular/entrained flow pattern could be described as dispersed. The aggravated dryout problem reported from heat transfer experiments at high mass flux could be explained by increased entrainment. Stratified flow was not observed in the tests with heat load. Bubble formation and growth could be observed in the liquid film, and the presence of bubbles gave differences in flow pattern compared to adiabatic flow. The flow pattern observations did not fit generalized maps or transition lines showed in the literature.

Two-phase flow boiling in a microfluidic channel at high mass flux

2020 ◽  
Vol 32 (9) ◽  
pp. 093309 ◽  
Author(s):  
Chanyoot Keepaiboon ◽  
Ahmet Selim Dalkilic ◽  
Omid Mahian ◽  
Ho Seon Ahn ◽  
Somchai Wongwises ◽  
...  
Keyword(s):  
Mass Flux ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Microfluidic Channel ◽  
High Mass ◽  
Phase Flow ◽  
Two Phase

Characteristics of Two-Phase Flow Pressure Drop of Propane in Horizontal Circular Small Tube

2016 ◽  
Vol 819 ◽  
pp. 371-375
Author(s):  
Agus Sunjarianto Pamitran ◽  
Sentot Novianto ◽  
T.A. Simanjuntak ◽  
Nasruddin ◽  
Muhammad Idrus Alhamid
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Mass Flux ◽  
Two Phase Flow ◽  
Saturation Temperature ◽  
Experimental Result ◽  
Phase Flow ◽  
Two Phase ◽  
Small Tube ◽  
Flow Pressure

This study experimentally investigated two-phase flow pressure drop of propane as refrigerant in horizontal small tube. Inner diameter and length of the tube were 7.6 mm and 1.07 m, respectively. In order to get pressure drop data, the experiment was conducted in various conditions of 10 to 25 kW m-2 heat flux, 200 to 628 kg m-2 s-1 mass flux, and 4.0 to 11.7°C saturation temperature. This study clearly showed the effect of heat flux, mass flux, and saturation temperature on the pressure drop of propane. This study also investigated which fluid properties gave higher effect on the frictional pressure drop due to its change over the process based on the recent experiment data. The existing pressure drop correlations were evaluated against the experimental result.

Experimental Study on Two-Phase Heat Transfer of FC-72 in Microchannels Heat Sink

2009 ◽  
Author(s):  
Xiao Hu ◽  
Guiping Lin ◽  
Hongxing Zhang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Flow Rate ◽  
Mass Flux ◽  
Heat Sink ◽  
Heat Fluxes ◽  
High Mass ◽  
Two Phase ◽  
Two Phase Heat Transfer

A closed-loop two-phase microchannels cooling system using a micro-gear pump was built in this paper. The microchannels heat sink was made of oxygen-free copper, and 14 parallel microchannels with the dimension of 0.8mm(W)×1.5mm(D)×20mm(L) were formed by electric spark drilling followed by linear cutting which separated the channels from each other. The heat transfer performance was evaluated by the fluid temperature, the pressure drop across the micro-channels and the volumetric flow rate. Experiments were performed with refrigerant FC-72 which spanned the following conditions: initial pressure of Pin = 73 kPa, mass velocity of G = 94 – 333 kg/m2s, outlet quality of xe,out = 0 – superheat and heat flux of q″= 25–140 W/cm2. The result showed that, the maximum heat flux achieved 96 W/cm2, as the heating surface temperature was kept below 85 °C and critical heat flux occurred in the condition of low flow rate. Average two-phase heat transfer coefficients increased with the heat flux at low mass flux (G = 94 and 180 kg/m2s) and all heat fluxes, high mass flux (G = 333 kg/m2s) and all heat fluxes, and moderate mass fluxes (G = 224kg/m2s) under low and moderate heat fluxes (q″<110 W/cm2 for G = 224 kg/m2s), which was a feature of nucleate boiling mechanism. Pressure drop through microchannels heat sink was found to be below 4kPa.

Pressure Drop of Two-Phase Flow Boiling with R-22 and R-290 in 7.6 mm Diameter Tube

2016 ◽  
Vol 818 ◽  
pp. 23-27
Author(s):  
Agus Sunjarianto Pamitran ◽  
Nasruddin ◽  
Helmi Dadang Ardiansyah ◽  
Muhammad Idrus Alhamid
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Mass Flux ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Saturation Temperature ◽  
Phase Flow ◽  
Lower Mass ◽  
Two Phase ◽  
Experimental Pressure

The characteristics of two-phase flow boiling of R-290 are required for replacing R-22 that has been phased-out. The present study focuses on experimental pressure drop for R-22 and R-290. The experiment was run with heat flux of 5.09 kW/m2 to 19.03 kW/m2, mass flux of 114.91 kg/m2s to 751.74 kg/m2s and saturation temperature of 4.77°C to 18.12°C. The present result showed that pressure drop was affected by heat flux, mass flux and saturation temperature. Lower mass flux, heat flux and saturation temperature results in lower pressure drop. The pressure drop of R-290 is lower than that of R-22. Among the existing pressure drop prediction methods, Lokhart-Martinelli (1949) gives the best prediction for the present pressure drop data.

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