Micro-channel flow boiling heat transfer of R-134a, R-236fa, and R-245fa

The rapid development of micro-thermal technologies has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes per unit volume, their heat transfer characteristics are still unclear and require investigation. The present study illustrates heat transfer data for flow boiling in a single micro-channel, for two channel diameters, namely, 510 and 790μm, three fluids, namely, R-134a, R-236fa and R-245fa, mass velocities from 300 to 2,000kg/m2s, and heat fluxes up to 200kW/m2. Stable flow boiling heat transfer data are analyzed through a parametric investigation, and are also confronted with measurements in the presence of two-phase oscillatory instabilities, which were found to significantly change the trends with respect to vapor qualit

Pressure drop prediction in annular two-phase flow in macroscale tubes and channels

A new prediction method for the frictional pressure drop in annular two-phase flow is presented. This new prediction method focuses on the aerodynamic interaction between the liquid film and the gas core in annular flows, and explicitly takes into account the asymmetric liquid film distribution in the tube cross section induced by the action of gravity in horizontal tubes operated at low mass fluxes. The underlying experimental database contains 6291 data points from the literature with 13 fluid combinations (both single-component saturated fluids such as water, carbon dioxide and refrigerants R12, R22, R134a, R245fa, R410a, R1234ze, and two-component fluids such as water-argon, water-nitrogen, alcohol-argon, water plus alcohol-argon and water-air), vertical and horizontal tubes and annuli with diameters from 3 mm to 25 mm, and both adiabatic and evaporating flow conditions. The new prediction method is very simple to implement and use, is physically based and outperforms existing pressure drop correlations (mean absolute error of 12.9%, and 7 points out of 10 captured to within +/- 15%). (C) 2016 Elsevier Ltd. All rights reserved

A micro particle shadow velocimetry (μPSV) technique to measure flows in microchannels

A micro particle shadow velocimetry (μPSV) system based on back-lit illumination and forward scatter observation of light from non-fluorescent particles has been developed. Relatively high luminous efficiencies and particle image contrasts were achieved by using the condenser stage of a standard transmitted light microscope and a continuous incoherent collimated light emitting diode (LED). This paper includes a critical review of the operating principles, benefits and practical problems associated with the predominant epifluorescent micro particle image velocimetry (μPIV) technique, and the less common light scatteringμPIV methods of whichμPSV is a development. ThisμPSV system was then successfully used to measure axial velocity profiles in a 280-μm-diameter circular channel up to a Reynolds number of 50 which corresponds to peak velocities of around 0.4 m/s. These velocity profiles were then integrated to provide instantaneous flow rates on the order of 100μl/min to an accuracy of±5% relative to average flow rates determined using a digital balance. Due to the incoherent nature of the LED light source, the back-lit forward scatter observation mode and the applied refractive index matching system, the location of the test section walls and thus the local velocity fields were also accurately obtained. As a result of this,μPSV provides a low cost and safe way to investigate microfluidics, especially in lab-on-a-chip applications where the necessary optical access through transparent test sections is often availabl

Assessing Food Security in Ethiopia with USDA ERS’s New Food Security Modeling Approach

Purpose: We analyze several dimensions of food security in Ethiopia, taking into account projected population growth, economic growth, and price information to estimate future food consumption by income decile. The analysis looks at the potential impact of large consumer price increases on food security metrics. Methodology: We use USDA ERS’s new modeling framework for its annual International Food Security Assessment. The modeling approach captures economic behavior by making food demand systematically responsive to income and price changes—a demand specification wellgrounded in microeconomic foundations. The projected change in food consumption can be apportioned to population growth, income growth, and changes in food prices and real exchange rates. Findings: Ethiopia is highly food-insecure, with 54% of the population (52 million people) consuming less than 2,100 calories a day in the base year (average 2013–15). Income growth under unchanged prices mitigates food insecurity with the number of food insecure people falling to 42.5 million in 2016. If domestic prices were free to fall with world market prices, the food insecure population would decrease further to 36.1 million. If domestic prices increased because of domestic supply shocks and constrained imports, the food-insecure population could rise to 64.7 million. The food gap (i.e., the amount of food necessary to eliminate food insecurity in the whole country) would reach 3.6 million tons. Implications: The current policy of promoting food security through autarky has some severe limitations. Allowing private traders to import food grains and hedge price variations and exchange rate changes, would greatly improve food security in Ethiopia

Fundamental issues, technology development and challenges of boiling heat transfer, critical heat flux and two-phase flow phenomena with nanofluids

This paper presents a comprehensive and critical review of studies on nucleate pool boiling heat transfer, flow boiling heat transfer, critical heat flux (CHF) and two-phase flow phenomena with nanofluids. First, general analysis of the available studies on the relevant topics is presented. Then, studies of physical properties of nanofluids are discussed. Next, boiling heat transfer, CHF phenomena and the relevant physical mechanisms are explored. Finally, future research needs have been identified according to the review and analysis. As the first priority, the physical properties of nanofluids have a significant effect on the boiling and CHF characteristics but the lack of the accurate knowledge of the physical properties has greatly limited the studies. Fundamentals of boiling heat transfer and CHF phenomena with Nanofluids have not yet been well understood. Flow regimes are important in understanding the boiling and CHF phenomena and should be focused on. Two phase pressure drops of nanofluids should also be studies. Furthermore, economic evaluation of the enhancement technology with nanofluid should be considered for the new heat transfer enhancement technology with nanofluids. Finally, applied research should be targeted to achieve an enabling practical heat transfer and CHF enhancement technology for engineering application with nanofluids

Prediction of flow boiling heat transfer of carbon dioxide inside small diameter tubes

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.This paper reviews recent experimental work on intube flow boiling of carbon dioxide and describes the updated flow boiling heat transfer model and flow pattern map proposed by Cheng, Ribatski, Wojtan and Thome [1] for evaporation of carbon dioxide in small diameter horizontal tubes. This new flow boiling heat transfer model predicted 76% of the CO2 database taken from the literature within ±30%. The new model and map are applicable to: tube diameters from 0.8 to 10 mm, mass velocities from 170 to 570 kg/m2s, heat fluxes from 5 to 32 kW/m2 and saturation temperatures from -28°C to 25°C (reduced pressures from 0.21 to 0.87). Simulations of the flow pattern map and heat transfer model are also shown.cs201

Role of Imaging Specrometer Data for Model-based Cross-calibration of Imaging Sensors

Site characterization benefits from imaging spectrometry to determine spectral bi-directional reflectance of a well-understood surface. Cross calibration approaches, uncertainties, role of imaging spectrometry, model-based site characterization, and application to product validation

Micro-channel flow boiling heat transfer of R-134a, R-236fa, and R-245fa

The rapid development of micro-thermal technologies has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes per unit volume, their heat transfer characteristics are still unclear and require investigation. The present study illustrates heat transfer data for flow boiling in a single micro-channel, for two channel diameters, namely, 510 and 790 mu m, three fluids, namely, R-134a, R-236fa and R-245fa, mass velocities from 300 to 2,000 kg/m(2) s, and heat fluxes up to 200 kW/m(2). Stable flow boiling heat transfer data are analyzed through a parametric investigation, and are also confronted with measurements in the presence of two-phase oscillatory instabilities, which were found to significantly change the trends with respect to vapor quality

Cooling of microprocessors using flow boiling of CO2 in a micro-evaporator : preliminary analysis and performance comparisons

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.No abstract available, please open full text articlevk201