Dry Out Research Articles

Experimental study of ammonia flow boiling in a vertical tube bundle: Part 4 – Comparative analysis

This paper is the last leg of the previous three papers of the same series with experimental study carried on a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes. Three different tube configurations were tested. (1) bare tube (2) with full length solid PVC nonconductive round rod insert (12.7 mm diameter) and (3) 33% cut insert. For the same flow, tubes with rods developed annular flow within the annular portion between the tube and the rod. Source of heat was hot ethylene glycol/water solution flowing on the shell side. Saturation temperature of ammonia was -20 °C ≤ Tsat ≤ -2 °C, heat flux range 5 kW m−2 ≤ q˙ ≤ 45 kW m−2 and mass flow rate of 0.008 and 0.015 kg s−1. Tests were conducted for each mass flow rate at four saturation temperature. Two-phase heat transfer coefficient increased with saturation temperature and heat flux for all cases with initial increase and then a drop at the onset of partial dry out. The tube with 33% cut rod exhibited better overall performance. Results were also compared with plate type evaporator data.

Dynamics of liquid film rupture under local heating

Rupture dynamics of a horizontal liquid film non-uniformly heated from below was experimentally studied in a wide range of liquid viscosity. To visualize the liquid surface deformations and disruption of the film, an optical schlieren system was used. The instantaneous local film thickness was measured using a chromatic confocal sensor. In order to measure the thickness of thin liquid films, the substrate was covered with a black coating eliminating reflection of the incident light. Angular and frequency characteristics of the confocal sensor were studied in detail. The process of film rupture can be divided into three stages: 1) film thinning down to a residual film on the heater; 2) existence of a stable residual film for some time; 3) rupture and dry out of the residual film. The thickness of the residual film was found to strongly depend on the liquid viscosity: for water it is about 10 µm, whereas for silicone oil PMS-200 it is about 275 µm. The data obtained for various working liquids were generalized by a dimensionless correlation.

Extending lifetime of gas-phase odor biosensor using liquid thickness control and liquid exchange

In recent few years, researchers utilized cell expressing olfactory receptor for vapor detection under various sensing mechanisms. Those olfactory systems, however, have relatively short lifetime due to the dry out of aqueous solution covering the cell. In this paper, we came up with a feedback control structure composed of an impedance measurement circuit, a microcontroller and two syringe pumps for maintaining thin liquid layer above cell. Cell lifetime was improved from less than 40 min to longer than 75 min when liquid film control was introduced. However, the biosensor lifetime remained similar between with or without liquid thickness control. Then, we added liquid exchange to further extend the lifetime of our odor biosensor. Minimal liquid exchange speed was able to significantly extend the biosensor lifetime. Meanwhile, faster liquid exchange speed resulted in better sensor responses. Furthermore, the enhancement acquired from intermittent liquid exchange was compared with continuous one. In this study, the lifetime of odor biosensor was extended to more than 3 h whereas it was less than half an hour without liquid thickness control. We believe the methodology we established in this paper will facilitate gas phase odor biosensor in continuous monitoring of target substances.

Achieving ultra-high coefficient of performance of two-phase microchannel heat sink with uniform void fraction

Thermal management of high-power electronics/devices has been a very challenging issue for data centers, flourishing electric vehicle revolution as well as cooling of the first wall of a Tokamak fusion reactor. As a prominent cooling strategy, two-phase flow in microchannels has received extensive research over decades. However, inherent defects such as relatively low critical heat flux due to the dry out of liquid film near the exit of the channel and relatively high two-phase flow pressure drop at high heat flux still hinder extensive commercial applications. The present work proposes a robust microchannel heat sink design toward uniform void fraction distribution along the flow direction. The two-phase flow pressure drop will not increase significantly with increase in heat flux. This is made possible by an innovative and unique combination of diverging microchannel and counter-flow manifold, which enables extensive channel-to-channel heat transfer, especially near the end or inlet of the channels. Moreover, flow visualization reveals that nearly uniform void fraction distribution along the channel with relatively high bubble slug ratio is possible. For a relatively large area of 12 cm2 and under the limitation of wall temperature of 140 °C, a heat flux as high as 3525 kW/m2 is achieved without sign of reaching the critical heat flux with nearly negligible pressure drop increment comparing to single-phase convection for a relatively low mass flux of 600 kg/m2s. An ultra-high coefficient of performance of 75,675 is attained under a low inlet temperature and low mass flux.

Experimental study of ammonia flow boiling in a vertical tube bundle: Part 3 – Enhanced dimple tube with 2/3rd height solid round PVC nonconductive rod

This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes with solid PVC nonconductive round rods inserted in the lower 2/3rd section of each tube creating 67% annular section. The source of heat was hot ethylene glycol/water solution flowing on the shell side. The range of saturation temperature for ammonia was -20 °C ≤ Tsat ≤ -2 °C, heat flux range 5 kW m−2 ≤ q˙ ≤ 45 kW m−2, exit vapor quality range 0.1 ≤ x ≤ 0.9 and mass flux 76.9 kg m−2 s−1≤ G ≤ 145.7 kg m−2 s−1. For each saturation temperature, the effects of heat flux and mass flux were studied while the exit vapor quality at each mass flux varied between 0.1 and 0.9. Around 40% increase in heat transfer performance was observed with saturation temperature between –20 and –2 °C at exit vapor quality of 0.7. Heat transfer performance also increased with heat flux at fixed mass flux at each saturation temperature while the performance showed initial increase with quality and then a drop at the onset of dry out.

Experimental study of ammonia flow boiling in a vertical tube bundle: Part 1 – Enhanced dimple tube

• Boiling of ammonia in a vertical shell and tube evaporator with enhanced dimpled tube bundle. • Study of wide range of experimental parameters. • Enhanced heat transfer coefficient compared to previous studies. This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes. The tests were conducted for a wide range of parameters. The heat load was provided by hot ethylene glycol/water solution flowing on the shell side. Saturation temperature of ammonia was -20°C ≤ T sat ≤ 2°C, heat flux range 5 kW m −2 ≤ q ˙ ≤ 45 kW m −2 , exit vapor quality range 0.1 ≤ x ≤ 0.9 and a mass flux range of 34.99 kg m −2 s −1 ≤ G ≤ 66.32 kg m −2 s −1 . For each saturation temperature, the effects of heat flux and mass flux were studied for the saturated temperature range while the exit vapor quality at each mass flux was varied between 0.2 and 0.9. Two phase heat transfer coefficient increased with saturation temperature and heat flux. For all experimental conditions, performance showed initial increase and then a classic drop at the onset of dry out. The dimple tube showed better heat transfer coefficient compared to the previous studies performed on plain tubes with ammonia.

Experimental study of ammonia flow boiling in a vertical tube bundle: Part 2 – Enhanced dimple tube with full length solid round PVC nonconductive rod

• Boiling of ammonia in a vertical dimpled tube bundle with round PVC nonconductive rods. • Study of wide range of experimental parameters. • Enhanced heat transfer coefficient compared to previous studies. This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes with solid PVC nonconductive round rods inserted in each tube creating an annular section. The source of heat was hot ethylene glycol/water solution flowing on the shell side. The range of saturation temperature for ammonia was –20 °C ≤ T sat ≤ –2 °C, heat flux range 5 kW m −2 ≤ q ˙ ≤ 45 kW m −2 , exit vapor quality range 0.1 ≤ x ≤ 0.8 and mass flux 76.9 kg m −2 s −1 ≤ G ≤ 145.7 kg m −2 s −1 . For each saturation temperature, the effects of heat flux and mass flux were studied while the exit vapor quality at each mass flux varied between 0.1 and 0.8. Two phase heat transfer coefficient increased with saturation temperature and heat flux. For all test runs, typical thermal performance was observed, i.e., initial increase and then a drop at the onset of dry out. Annular flow boiling showed better heat transfer coefficient compared to the dimple tube for quality x < 0.5.

Investigation of Gas Diffusion Layer and Microporous Layer Effect on Water and Thermal Management in Hydroxide Exchange Membrane Fuel Cells

Hydroxide Exchange Membrane Fuel Cells (HEMFC) are continually being researched as a potentially more cost-effective alternative to Proton Exchange Membrane Fuel Cells (PEMFC). In the high pH environment in HEMFCs, platinum group metal free catalysts have shown promising results, and bipolar plates can be manufactured using more cost-effective material1. These aspects can significantly reduce the overall cost; however, improvements in HEMFCs performance and durability are needed for commercialization.A key challenge in the design of PEM and HEM fuel cells is optimizing water and thermal balance to achieve maximum performance and durability. Water and thermal management are extremely important to understand and optimize in HEMFCs due to their influence on kinetic, ohmic, and transport overpotentials2. Significant efforts in water management have been made in PEMFCs in order to achieve higher performance and durability, specifically using a microporous layer (MPL) to optimize oxygen transport in the cathode where water and heat are generated3. We can use knowledge gained from PEMFC in HEMFCs; however, our water generation and movement are flipped, as shown in Figure 1. In PEMFCs the cathode generates 2 water molecules per 4 electrons, and in HEMFCs the anode produces 4 water molecules, and the cathode consumes 2 water molecules per 4 electrons. The water consumption in the cathode along with the heat generation increases the risk of dry-out. On the other side, the anode produces water and could possibly lead to flooding. Furthermore, the goal in HEMFCs, in order to achieve better performance and durability, is to use the water produced in the anode to help hydrate the membrane and cathode ionomer to minimize dry-out without causing flooding losses4.Our current investigation is to manipulate the water and thermal management through the gas diffusion layers (GDLs). An MPL can be added to influence liquid water within the cell, specifically on the anode. Figure 2 (a) shows four identical cells with different GDL combinations, 29BC is with an MPL and 25BA is without an MPL. At high temperature and flowrates, the capillary pressure from liquid water in the anode catalyst layer does not exceed that needed to enter the MPL and, therefore, hydrates the membrane. Other benefits from the MPL includes improved contact between the GDL and catalyst layer and higher electronic conductivity. Another aspect of GDLs that can manipulate water management is through altering the thermal conductivity. Figure 2 (b) shows three identical cells with Sigracet (SGL) and Toray GDLs, where Toray thermal conductivity is 4 times higher than SGL. Higher thermal conductivity decreases temperature gradients within the MEA and can decrease risk of dry out. But lower temperature gradients could lead to less liquid water vaporization within the catalyst layer and increases risk of flooding. We report the MPL is beneficial between the anode catalyst layer and GDL to promote back diffusion, and the temperature gradients effected by the thermal conductivity have a significant impact on water content in the cathode. Setzler, B. P., Zhuang, Z., Wittkopf, J. A. & Yan, Y. Activity targets for nanostructured platinum-group-metal-free catalysts in hydroxide exchange membrane fuel cells. Nat. Nanotechnol. 11, 1020–1025 (2016).Weber, A. Z. & Newman, J. Coupled Thermal and Water Management in Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 153, A2205 (2006).Weber, A. Z. & Newman, J. Effects of Microporous Layers in Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 152, A677 (2005).Omasta, T. J. et al. Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs. Energy Environ. Sci. 11, 551–558 (2018). Figure 1

Paper-based aqueous Al ion battery with water-in-salt electrolyte

Low-cost, flexible and safe battery technology is the key to the widespread usage of wearable electronics, among which the aqueous Al ion battery with water-in-salt electrolyte is a promising candidate. In this work, a flexible aqueous Al ion battery is developed using cellulose paper as substrate. The water-in-salt electrolyte is stored inside the paper, while the electrodes are either printed or attached on the paper surface, leading to a lightweight and thin-film battery prototype. Currently, this battery can tolerate a charge and discharge rate as high as 4 A g−1 without losing its storage capacity. The charge voltage is around 2.2 V, while the discharge plateau of 1.6–1.8 V is among the highest in reported aqueous Al ion batteries, together with a high discharge specific capacity of ∼140 mAh g−1. However, due to the water electrolysis side reaction, the faradaic efficiency can only reach 85% with a cycle life of 250 due to the dry out of electrolyte. Benefited from using flexible materials and aqueous electrolyte, this paper-based Al ion battery can tolerate various deformations such as bending, rolling and even puncturing without losing its performance. When two single cells are connected in series, the battery pack can provide a charge voltage of 4.3 V and a discharge plateau as high as 3–3.6 V, which are very close to commercial Li ion batteries. Such a cheap, flexible and safe battery technology may be widely applied in low-cost and large-quantity applications, such as RFID tags, smart packages and wearable biosensors in the future.

The Dynamic of toxic in acid sulphate soil, planting rice in Mekong Delta, Caused by Natural environmental factors

Through series of experiments with 4 iterations, with statistical processing of reliability, we see: Toxicity in the environment in acid sulfate soil has very large and complex, this variation depends: 1-To dry out, the amount of toxins increases, which can cause rice death.

Experimental investigations on two-turn cryogenic pulsating heat pipe with cylindrical shell-type condenser

• Cryogenic Pulsating Heat Pipe is experimentally investigated. • A cylindrical shell-type condenser is proposed for Liquid nitrogen-based Cryogenic PHP. • The influence of heat input and Filling Ratio (FR) on the performance of Cryogenic PHP is studied. • Fast Fourier Transform (FFT) analysis is carried out to comprehend the Cryogenic PHP system. Pulsating heat pipe (PHP) is a two-phase passive cooling device and has become an efficient heat transport system to replace bulky conductive straps in cryogenics applications. A two-turn cryogenic PHP is experimentally investigated with liquid nitrogen as working fluid in the present paper. A cylindrical shell-type condenser is proposed that provides a high cooling load with easy refilling of liquid nitrogen. The inner diameter of the capillary tube is 1.3 mm, and the outer diameter is 3.2 mm. The filling ratios are kept as 38%, 57%, 76%, and 95%. Heat input is varied in the range of 1 W-80 W. The maximum heat transport capacity of a two-turn cryogenic PHP with cylindrical shell-type condenser is observed 70 W before dry out. The maximum effective thermal conductivity ( K eff ) of 16350 W/m-K is observed at an optimum filling ratio of 76%. Fast Fourier Transform (FFT) analysis showed cryogenic PHP's deterministic behaviour at the optimal stage with a frequency range of 0.62–1.56 Hz.

A numerical study of slug bubble growth during flow boiling in a diverging microchannel

The diverging microchannel has a gradually expanding cross-section area along the flow direction that can reduce flow instability and hence partial dry out. Utilizing the VOF method, Hardt and Wondra phase change model, and dynamic refined mesh scheme within a self-developed OpenFOAM solver, the vapor slug bubble’s growth inside diverging and uniform microchannels are investigated. The effects of mass flux, heat flux, and diverging gradient on the bubble behavior and flow instability are discussed. The bubble growth caused the upstream flow to slow down and downstream flow to speed up, which lead to the pressure drop oscillation. The velocity of the flow is higher in the diverging microchannel cases, either upstream or downstream the vapor slug bubble. These indicate the expanding cross-section area can promote the bubble to move toward the downstream outlet thus mitigate the blockage of the channel and flow instability. HIGHLIGHT The effects of slug bubble growth on flow and heat transfer are revealed through numerical investigation. The bubble growth process varied in the diverging and uniform microchannels. The diverging channel can help mitigate the blockage of the microchannel and flow instability.

CLT construction without weather protection requires extensive moisture control

This study examines how cross-laminated timber (CLT) constructions, including joints, connections and attachment points, are affected by precipitation during construction. The case studies are based on moisture content measurements and material sampling as well as microbiological analysis during the structure’s construction stage. The study does not include remediation control. The field measurements show microbiological growth in all buildings and almost all floor structures that were investigated. Of a total of 200 measuring points analysed, half had mould growth and around a third had moderate or extensive growth. The moisture content measurements for one of the locations with the largest percentage of elevated or high moisture content was at the top of the floor structure in the bottom gap between timbers in the CLT top layer. This is one example of several materials or construction components where there is limited possibility of dry out. Based on the outcome, it would appear difficult, or impossible, to avoid the appearance of microbial growth during construction with CLT without weather protection. Previous studies indicate that microbiological analysis of CLT is extremely rare in both laboratory and field studies, which implies that there are obvious shortcomings in the scientific work. The fact that mould growth is often invisible needs to be disseminated, especially in practical studies. However, there seems to be a good level of awareness in the literature that theoretical studies often conduct mould growth risk evaluations. There do not appear to be any moisture safety assembly methods or solutions for CLT construction that do not have weather protection or a declaration of the critical moisture conditions for CLT products.

Key Factors and Methods for Successful Live Transport of Wild Caught Shrimp (Pandalus borealis)

ABSTRACT The Northern shrimp (Pandalus borealis) is an important fishery resource of the Northern hemisphere. Caught by large offshore trawlers, this species is mainly landed as a cooked and frozen product. However, for boats fishing close to the shore, the landing of live shrimp could be a preferable option, as higher prices could be attained for this type of product. We hypothesize that landing and transport of live shrimp is feasible, if key factors for survival and suitable transportation methods are identified. To increase our understanding of post-harvest handling and its effect on shrimp survival, we conducted a set of dry (out of water) and wet (in water) transport experiments. Different cooling methods and temperatures were tested as well as the use of the sedative AQUI-S®. Additionally, a set of pilot studies was conducted to determine the time frame for post-harvest recovery and to identify a favorable water temperature to ensure optimal revitalization. A post-transport recovery period of 48 hours at water temperatures below 8°C was found to be appropriate and was applied to experimental animals before simulating different transport conditions. Live shrimp, transported dry for 24 hours at −0.5°C, showed a significantly lower mortality rate (14.4%) compared to shrimp transported dry at 2°C (67.8%). Transport of shrimp in water (3.0°C) at two different densities (~80 and ~165 kg/m3) did not reveal any significant difference in mortality after a 4-day period, with an overall survival of 92.5%. Although survival was not improved by post-transport treatment with AQUI-S®, our results show that it is possible to deliver vital live shrimp to the market, given optimal post-harvest revitalization and careful choice of transportation method.

Experimental investigation of the annular flow caused by convective boiling in a heated annular channel

An experimental investigation was performed on the convective boiling heat transfer of the annular flow in concentric and eccentric annuli with a central heating rod in an unheated tube. Advanced measurement techniques including laser-induced fluorescence (LIF) and confocal chromatic sensor (CCS) were applied to acquire the dynamics and instabilities of the liquid thin film on the tube. The boundary conditions were as follows: heating rod heat flux from 167 kW/m2 to 201 kW/m2 and mass flow rate from 58 g/s to 155 g/s. The annuli which the flow occurred has a hydraulic diameter of 15.5 mm. As indicated previously by the research on isothermal annular flow in bare tubes, the vapor superficial velocity is the primary factor that influences the liquid film dynamics including the base film thickness and wave amplitude. The liquid film thickness in the eccentric geometry was observed to be constant for liquid superficial velocities from 0.15 m/s to 0.34 m/s and vapor superficial velocities from 6.5 m/s to 13.2 m/s. The heat transfer coefficient ranged from 2.734 kW/m2∙K to 4.279 kW/m2∙K for the concentric geometry and 2.063 kW/m2∙K to 3.096 kW/m2∙K for the eccentric geometry. An increasing trend of the heat transfer coefficient was observed as the liquid superficial velocity increased, whereas the reverse trend was observed for the vapor superficial velocity. When the measured heat transfer coefficient was exceptionally low (< ~2.0 kW/m2∙K), the boiling condition was assumed to be the dry out. An extended range of boundary conditions reported in this study (from the annular flow to the dry out regime) can provide useful data sets for validating computer codes. In particular, the detailed information on liquid film thickness, wave characteristics, and heat transfer coefficient provided in the present paper can help develop annular flow boiling models in computational fluid dynamics.

An Innovative Method for the Theoretical Investigation on the Critical Heat Flux of Nanofluid Flow Boiling With Nanoparticle Deposition

As a complex thermal critical phenomenon, CHF is widely considered in power system. Nanofluid is regarded as a new potential work fluid because of its excellent heat transfer capacity, the investigation on the CHF of nanofluid boiling is requisite and significative. In present work, the theory about the adsorption of nanoparticle porous deposition layer is proposed and the adsorption force has been analyzed and derived. The force balance model of steam bubble block during nanofluid flow boiling process is established including axial and radial forces. In addition, a code of nanofluid flow boiling is compiled based on the CHF theoretical model of liquid sublayer dry out. After that, the code is validated by the experimental results for the CHF of nanofluid flow boiling under the condition of low mass flux and low pressure. Then, the code is used to analyze the CHF of nanofluid flow boiling under the condition of different mass flux and nanoparticle diameter. The results indicates that the CHF of nanofluid flow boiling can be increased more significantly under the condition of greater mass flux, less nanoparticle diameter and others which can cause the outlet quality to be increased like greater inner diameter and less heating length.

Cutaneous candidiasis mimicking inverse psoriasis lesion in a type 2 diabetes mellitus patient

Background:&nbsp; Type 2 diabetes mellitus (T2DM) condition could affect the skin. The disease is among the greatest risk factors of skin infection such as cutaneous candidiasis and is also associated with autoimmune skin diseases such as psoriasis, which lesion morphologies and predilection areas are quite similar. These similarities are a source of confusion to clinicians. Case illustration:&nbsp; We present a 60-year-old female patient with uncontrolled T2DM and multiple erythematous plaques in the form of shiny lesions at the intertriginous area that developed 1 week prior to her hyperglycemic state. The lesions had recurred at the same site for approximately 10 years and were initially diagnosed as&nbsp; intertriginous candidiasis. Bedside testing revealed a positive Auspitz sign, while the patient&rsquo;s fungal culture&nbsp; was unremarkable. She was diagnosed with inverse psoriasis (IP) and treated with 0.1% mometasone furoate cream twice a day. The lesions improved but were not completely resolved. Discussion:&nbsp; Inverse psoriasis is a subgroup of psoriasis characterized with thin, non-layered scales, and various predilection areas. The features of the primary l esion may be altered by a patient&rsquo;s activities, such as&nbsp; use of cleansing soap, which may cause lesions to dry out and ooze, similar to cutaneous candidiasis. Additional examination, such as bedside testing and laboratory work, could help obtain a proper diagnosis.&nbsp; Conclusion:&nbsp; The similar morphologies and predilection areas of IP and cutaneous candidiasis lesions may confuse clinicians. In some limited cases, the correct diagnosis may be obtained by complete history taking, physical examination, and other simple tests. Keywords : inverse psoriasis, intertriginous cutaneous candidiasis, type 2 diabetes mellitus Normal 0 false false false EN-US X-NONE X-NONE

Analysis of the natural circulation test of PHENIX reactor by the THACS code

For the pool type sodium-cooled fast reactor (SFR), whether the primary circuit can establish a stable natural cycle after the reactor accident, and whether the primary natural cycle can take away the decay heat of the reactor core in the long-term, are the key issues in the design and safety analysis of SFR. In order to verify the natural circulation capacity of the primary circuit, a natural circulation test was carried out on pool type SFR Phenix by CEA before the reactor is decommissioned. One objective of this test is to verify the system codes used to simulate sodium natural convection phenomena.In this paper, the system code THACS is used to analyze Phenix natural circulation test, and the capacity of THACS to analyze the natural circulation of SFR is evaluated. Firstly, this paper introduces the components of Phenix reactor and the process of natural circulation test. Then, the models of program and the computational modeling of Phenix reactor are described. The calculation involves the whole process of the test: dry out of steam generators, scram, stop of pumps, two phases of natural circulation. Then, the results are compared with the test data, and the influence of inter-wrapper flow (IWF) on core outlet temperature is emphasized. The results show good applicability of THACS to analyze natural circulation in SFR, and the IWF can reduce the peak value of core outlet temperature at the beginning of natural cycle. Finally, the deviation between calculation and test results are analyzed, and the limitation of the models are evaluated.

Flow boiling heat transfer characteristics of R245fa refrigerant in a plate heat exchanger

Plate heat exchangers (PHEs) are used extensively in industrial applications and, owing to their compactness, and higher thermal efficiency, they are keys component of organic Rankine cycle (ORC) application. This study presents the experimental heat transfer characteristics during flow boiling of R245fa refrigerant with a commercial working fluid used in ORCs; inside brazed plate heat exchanger with chevron angle of 65 degree. The flow boiling heat transfer characteristics were measured with varying saturation temperatures, mass flux and heat flux, which range from 55.5 °C-61.8 °C, 15.5-17.4 kg m−2 s−1, and 6400-10120 W m−2, respectively. The experimental results showed that flow boiling heat transfer coefficient is dependent upon the heat flux and mass flux. The results showed that the heat transfer coefficient increases with the increase of heat flux, and then starts to decrease due to local dry out. However, at low mass flux, locally triggering dry out was more prominent than that high mass flux. The heat transfer coefficient showed to be sensitive to the change in the saturation temperature. Moreover, flow boiling heat transfer coefficient showed a linear relationship with mass flux of the refrigerant.

Flow boiling characteristics in different configurations of stepped microchannels

Geometrical modifications in microchannel geometry intended towards mitigation of flow instabilities generally results in increased pressure drop across the microchannel heat sink. To overcome this, stepped microchannels configurations with combination of narrow bottom (aspect ratio: AR = depth/width > 1) and wider top (AR < 1) part are investigated and their flow boiling performances have been compared with rectangular cross-section microchannels (RMC) having same hydraulic diameter (350 μm). The effect of varying wider cross-section on flow boiling instability and heat transfer performance have been explored for three configurations of stepped microchannels namely: stepped straight microchannels (SMC), stepped converging microchannels (SCMC) and stepped diverging microchannels (SDMC). These geometrical variants are fabricated on a 10 × 10 mm2 copper block having nine parallel microchannels. Flow boiling experiments have been conducted using de-ionized (DI) water as coolant for an inlet subcooling of 20 °C, coolant mass fluxes ranging from 327.6 to 777.54 kg/m2s and for wide range of heat fluxes. Combination of narrow and wider converging/diverging flow passage improves flow stability by allowing bubbles in the confined narrow bottom to grow and expand in the wider top section thus reducing their evaporative momentum. Different flow regimes govern the heat transfer and pressure drop mechanisms while the compressible volume-liquid interaction dictates flow instability. Favorable surface tension gradient in SDMC alleviates flow instability whereas reduced flow resistance at the inlet in SCMC resulted in smooth rewetting during dry out. Amplitude and fluctuation of pressure drop is found to be minimum in SCMC among all geometric configurations, while better heat transfer performance has been observed in SDMC configuration.