Experimental flow boiling study in a 0.52 mm diameter vertical tube using R134a

This article is also available online from the link below.Experimental results for R134a boiling on a vertical stainless steel tube of internal diameter 0.52 mm are presented in this paper. They form part of an ongoing study of flow boiling in a wide range of small diameter tubes. Other parameters were varied in the range: mass flux 300-700 kg/m2.s; heat flux 1.6-75 kW/m2 and pressure 6 -10 bar. The flow patterns visualised at the exit of the heated test section are first presented. The variation of the heat transfer coefficient with thermodynamic quality and its dependence on heat flux, mass flux and system pressure is then presented and discussed

Bubble Growth Models in Saturated Pool Boiling of Water on a Smooth Metallic Surface: Assessment and a New Recommendation

Prediction of bubble growth rate is very important for the development of accurate models for bubble departure diameter and thus the heat transfer rates in nucleate boiling. This paper presents an evaluation study to the existing homogeneous and heterogeneous bubble growth models using our experimental data for bubble growth in saturated pool boiling of deionized water on a plain copper surface. The experiments were conducted at pressures 1, 0.5 and 0.15 bar and superheat in the range 5.1 – 19.5 K. To start with, the paper presents a critical review on bubble growth models in homogeneous and heterogeneous boiling. It was found that homogeneous growth models achieved some partial agreement with the experimental data at some conditions and thus they should be used carefully in heterogeneous boiling. There was a good agreement between some of the models that were suggested based on the assumption that bubble growth occurs due to evaporation from the superheated boundary layer around the bubble. The models based on microlayer evaporation only could not explain the experimental data, i.e. partial agreement at some conditions. The model that predicted the data very well at all conditions was the “relaxation boundary layer” model by Van Stralen [25]. This model was generalized in the current study by suggesting two new empirical models for the departure diameter and departure time.Engineering and Physical Sciences Research Council of the UK, under Grant: EP/S019502/1

Flow boiling pressure drop of R134a in micro diameter tubes: Experimental results and assessment of correlations

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.This article has been made available through the Brunel Open Access Publishing Fund.The experimental results of two phase flow boiling pressure drop of R134a in vertical micro diameter stainless steel tubes are presented in this paper. The tests were conducted using four tubes; one tube with an inner diameter of 0.52 mm and 100 mm heated length and three tubes with an inner diameter of 1.1 mm and different heated lengths (150, 300 and 450 mm). Other experimental conditions include: mass flux range of 200 – 500 kg/m2 s, system pressure range of 6 – 10 bar, inlet sub-cooling of about 5K and heat flux range of 1 – 140 kW/m2. The results indicated that the total measured two phase pressure drop increases with increasing mass flux, heat flux (exit quality) and decreasing system pressure and tube inner diameter. The test section heated length was found to have a significant effect on the measured pressure drop per metre length. The total measured two phase pressure drop results were also compared with eighteen macro and micro scale models and correlations

Effect of different concentrations of phenol on growth of some fungi isolated from contaminated soil

Phenol concentration in 25 water samples collected from three Egyptian Governorates (El- Gharbia, Kafer El-Sheik and El-Menofia) was assayed. The wastewater collected from El-Mehalla El-Kobra II (El- Gharbia governorate) was the most polluted sample with phenol and was equal to 0.0 88 mg/L. Czapeks medium was the most suitable among the other tested media for the growth of Hormodendrum bergeri, Fusarium oxysporum and Aspergillus flavus var. coulmnaris. However, where they were able to grow in the media containing 0.1 g/100 ml phenol, they failed to grow in the potato dextrose medium (PDA) with 0.14 g/100 ml phenol. On the other hand, the efficiency of Aspergillus ochroceus to grow on phenol was low when compared with H. bergeri, F. oxysporum and A. flavus var. coulmnaris. The growth of H. bergeri, F. oxysporum and A. flavus var. coulmnaris was optimum on the medium that contained 0.1 g/100 ml phenol after 6 days. The addition of a mixture of vitamins (B1 + B6 + B12) at 0.1% (w/v) to Cazpeks medium enhanced the growth of H. bergeri, F. oxysporum and A. flavus var. coulmnaris in the presence of phenol. Growth in the presence of phenol induced some morphological modification in both F. oxysporum and A. flavus var. coulmnaris.Key words: Phenol, growth, fungi, morphological changes

Improvement of nutritional quality and antioxidant activities of yeast fermented soybean curd residue

This study evaluated the potential use and improves the health beneficial properties of the waste of soybean manufacturing products by solid-state fermentation of six GRAS different yeast strains, including extractable antioxidant activities and bioavailable nutritional compositions. In comparison with non-fermented okara (control), some levels of value addition occurred as a result of the fermentation. The protein contents increased by 20.10- 54.40%, while the crude fibre decreased by 7.38- 45.50% with different strains. With all the organisms used, the ash content increased while the carbohydrate and lipid contents were reduced. Total phenolic content and all parameters of antioxidative activities were increased in fermented substrate. The highest significant levels of antioxidant activities were achieved with Kluyveromyces marxianus NRRL Y-8281. Results showed that the nutritional quality and antioxidant activities of the substrate were enhanced by solid yeast treatment fermentation. Thus, scope exists for microbial upgrading of this low-quality waste and development of healthy animal feed supplements.Key words: Solid state fermentation, yeast, waste soybean manufacturing  products, antioxidant, protein, fiber

Pool boiling review: Part II – Heat transfer enhancement

Copyright © 2021 The Author(s). Heat transfer enhancement by surface modification has been extensively studied in the last twenty years. However, there remains a large discrepancy among researchers on the performance of enhanced surfaces even for the same fluid and surface preparation technique. The reasons of this discrepancy are not understood and are not discussed in past papers, including paper reviews. Part II of this two-part paper aims to present a detailed assessment of pool boiling heat transfer enhancement, relating this to Part I [1], which presented a critical assessment of fundamental concepts of heterogeneous nucleation. Current challenges in evaluating the performance of enhanced surfaces is first discussed. The performance of smooth and roughened surfaces is then discussed and the effect of fluid type is explained. Pool boiling data of two fluids, namely water and FC-72, on two enhanced substrate materials, i.e. copper and silicon were digitized and assessed in order to elucidate the reason for the discrepancy in published works and present future recommendations for heat transfer enhancement. The heat transfer enhancement mechanisms adopted by researchers were presented and critically discussed and compared. The paper contributes to the understanding of the effect of fluid-surface combinations and suggest guidelines for researchers to consider when evaluating the performance of enhanced surfaces. This will help the research community and industry to conclude on the best surface structure and surface manufacturing technique matching particular fluid of interest.Engineering and Physical Sciences Research Council, UK (Grant Reference: EP/S019502/1)

Flow Boiling in Micro-Passages: Developments in Fundamental aspects and Applications

Flow boiling in mini to micro passages located at the heat source, and as part of a thermal management system, has been identified as a possible way to remove the increasing high heat fluxes generated by high power electronic devices due to their capability of high heat transfer rates with small surface temperature variations. However, some still unresolved fundamental issues hinder the possible full adoption of this technology. These relate to the prevailing flow patterns, heat transfer rates and pressure drop in such geometries, and their dependence on key parameters. The possible major applications of flow boiling in microchannels are first mentioned in this paper, highlighting the requirements and the challenges of the thermal management of each application. The paper then presents new experimental research by the present authors as well as research reported in the literature on flow boiling in single tubes and rectangular multi microchannels to help elucidate the following fundamental issues: the definition of a microchannel, prevailing flow patterns, heat transfer mechanisms, flow instability and reversal and their effect on heat transfer rates, effect of channel material and surface characteristics (including latest research in coatings), effect of different fluid properties, and its relation to channel material, effect of channel length and aspect ratio. An appreciation of the above can help explain the interpretation of the prevailing fluid flow and heat transfer phenomena and the data scatter and discrepancies observed in past studies. In addition, models and correlations predicting flow patterns and heat transfer rates are presented

Bubble growth on a smooth metallic surface at atmospheric and sub-atmospheric pressure

Data availability: Data will be made available on request.Copyright © 2023 The Author(s). Bubble growth rate is one of the most important parameters required for the development of accurate mechanistic nucleate boiling heat transfer models. It is also very important for understanding the hydrodynamic forces and the mechanism of bubble departure. This paper presents an experimental study on bubble growth measurements in saturated pool boiling of deionized water on a plain copper surface at atmospheric and sub-atmospheric pressure. The measurements were conducted using a high-speed, high-resolution camera with a microscopic lens. The mechanisms of bubble growth are discussed, while the microlayer evaporation mechanism has been evaluated and discussed using the measured bubble growth curve. The estimated contribution of microlayer evaporation to a single bubble growth is about 70 %, while the contribution of latent heat transfer (evaporation) to the total heat transfer rate from the surface is about 30 %. The remaining 70 % is due to other mechanisms, i.e. conduction and convection. These values were obtained based on the analysis of the bubble growth curve only and agreed with some researchers who conducted local heat transfer measurements using integrated sensors or infrared thermography. These detailed measurement techniques cannot be used with the thick copper block tested in the current study, which was also tested by many researchers in literature and is representative of industrially used surfaces. It was also found that the bubble departure mechanism at atmospheric pressure is due to a static balance between surface tension and buoyancy forces while at sub-atmospheric pressure, it was between buoyancy and liquid inertia forces. The pressure did not have a significant effect on the characteristics of the dynamic contact angle, which was also measured from the instantaneous images of the bubble. It was concluded also that the force balance required for the accurate prediction of departure diameter should be conducted when the two forces are equal, which occurred at time less than the departure time and dynamic contact angle of about 45. In most bubble departure models, researchers recommended the balance to be conducted at the moment of departure when the bubble forms a neck with contact angle of 900 (underestimation to the surface tension force). The analysis of one of the commonly used homogeneous growth models indicated that for homogeneous bubble growth models to be applicable in nucleate boiling, an allowance must be made for the fact that the degree of superheat varies with time during a bubble growth period.Engineering and Physical Sciences Research Council of the UK, under Grant: EP/S019502/1

Pool boiling review: Part I – Fundamentals of boiling and relation to surface design

Copyright © 2021 The Author(s). The pool boiling process is one of the most effective heat transfer modes capable of transferring large amounts of heat with small temperature difference between the heated surface and the fluid. In addition, fundamental knowledge of pool boiling processes is the starting point of flow boiling research and applications. It is therefore no surprise that it has been, and still is, the subject of extensive research globally for quite some time and a critical analysis is now required in order to move forward with enhanced surface designs. The current on-going research focuses on the understanding of boiling fundamentals including bubble generation, growth and bubble dynamics. In this context, fluid-surface interaction is critical. In the first part of this two-part paper we present the factors and parameters affecting the above, starting with the criteria for gas/vapour entrapment, nucleation site stability and the superheat required for heterogeneous nucleation. The models predicting the incipience superheat are critically described, classified into phase instability and superheated boundary-layer based models. This first part includes bubble growth and departure models, elucidating the effect of surface topology and wettability that can inform and facilitate the design of enhanced surfaces that are presented in Part II [10]. Three fluids of industrial interest, i.e. FC-72, HFE7100 and water were used through the discussion, as examples, to represent low and high surface tension fluids and help the understanding of surface-fluid interactions and relation to possible heat transfer enhancements.Engineering and Physical Sciences Research Council, UK (Grant Reference: EP/S019502/1)