Saturated flow boiling in small- to micro- diameter metallic tubes: Experimental results and modeling

Some results of a long-term study of flow boiling patterns, heat transfer rates and pressure drop of R134a at pressures of 6-14 bar in five vertical stainless steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1 and 0.52 mm are presented in this paper. The flow regimes in the 4.26 mm to 1.1 mm tubes were identified as dispersed bubble, bubbly, slug, churn, annular and mist flows. As the diameter was reduced, progressively slimmer vapour slugs, a thinner liquid film around the vapour slug and a less chaotic vapour-liquid interface in churn flow were observed. Confined flow appeared first in the 2.01 mm tube. Dispersed bubble flow was not observed in the smallest tube (0.52 mm) for the range studied in runs in which wavy film flow occurred. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapour quality for low quality values. At higher quality, the heat transfer coefficients decreased with quality, indicating local dryout. The heat transfer characteristics of the 0.52 mm tube were different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. The pressure drop and heat transfer results were compared with existing correlations but with some limited success. Recent progress on mechanistic models for heat transfer along with comparisons and recommendations are included in the paper

A comparison with the three-zone model for flow boiling heat transfer in small diameter tubes

Flow boiling heat transfer experimental results, obtained in two stainless steel tubes of internal diameter 4.26 mm and 2.01 mm using R134a as the working fluid, indicate that the local heat transfer coefficient increases with heat flux and is independent of vapour quality when this is less than about 40% to 50% for the 4.26 mm tube and 20% to 30% for the 2.01 mm tube, conventionally interpreted as nucleate boiling. Above these quality values, the separate graphs merge into a single line for heat transfer coefficient decreasing with increasing vapour quality. The data in the apparently-nucleate boiling condition are compared with a recent state-of-the-art three-zone evaporation model for the confined bubble flow regime without a nucleate boiling contribution. The model predicts the experimental data reasonably well but does not predict correctly the trends for changing pressure and diameter. Some suggestions are made for improving the model. The comparisons made in this paper support the statements by the developers of the model and others that the application of conventional macro flow boiling correlations to micro tube flow boiling heat transfer may not necessarily have a sound physical basis

Flow boiling in a 1.1mm tube with R134a: Experimental results and comparison with model

A detailed comparison of the three-zone evaporation model, proposed by Thome et al. (2004), with experimental heat transfer results of two stainless steel tubes of internal diameter 4.26 mm and 2.01 mm using R134a fluid was presented by Shiferaw et al. (2006). In the current paper the comparison is extended to flow boiling in a 1.1 mm tube using R134a as the working fluid. Other parameters were varied in the range: mass flux 100-600 kg/m2.s; heat flux 16-150 kW/m2 and pressure 6-12 bar. The experimental results demonstrate that the heat transfer coefficient increases with heat flux and system pressure, but does not change with vapour quality when the quality is less than about 50% for low heat and mass flux values. The effect of mass flux is observed to be insignificant. For vapour quality values greater than 50% and at high heat flux values, the heat transfer coefficient does not depend on heat flux and decreases with vapour quality. This could be caused by partial dryout. The three-zone evaporation model predicts the experimental results fairly well, especially at relatively low pressure. However, the partial dryout region is highly over-predicted by the model. The sensitivity of the performance of the model to the three optimized parameters (confined bubble frequency, initial film thickness and end film thickness) and some preliminary investigation relating the critical film thickness for dryout to measured tube roughness are also discussed

Flow patterns and heat transfer for flow boiling in small to micro diameter tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1 and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m2s and the heat flux from as low as 1.6 to 135 kW/m2. Five different pressures were studied, namely 6, 8, 10, 12 and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapour quality for low quality values. At higher quality, the heat transfer coefficients decreased with quality, indicating local dryout. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tube higher. The heat transfer results suggested that a tube size of about 2 mm might be considered as a critical diameter to distinguish small and conventional tubes, This is consistent with an earlier study of flow patterns, in which confined bubble flow was observed only in the 2.01 and 1.1 mm tubes. Further differences have now been observed in the 0.52 mm tube: ring flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, both in flow patterns and heat transfer, indicate a possible second change from small to micro behaviour at diameters less than 1 mm for R134a

Boiling two-phase pressure drop in small diameter tubes

An experimental study of two-phase pressure drop in small diameter tubes is described in this paper. Stainless steel tubes of internal diameter and length of 4.26 mm, 500 mm and 2.01 mm, 211 mm were used. The working fluid was R134a and the range covered was: mass flux 100 – 500 kg/m2s; system pressure 8-14 bar and exit quality up to 0.9. The heat flux applied to the tubes ranged from 13 – 150 kW/m2. The effect of diameter on pressure drop is discussed in this paper and a detailed presentation of the results of the comparison with existing pressure drop correlations, some particularly developed for small tubes, is given

Levels and Trends of Occupational Hazards among Surgical Residents at Tikur Anbessa Hospital, Addis Ababa Ethiopia

Background: A previous study conducted four years ago among surgeons-in-training at the Addis Ababa University revealed that work-related  accidents among surgical trainees were enormous, and there was huge under reporting to the occupational health unit (OHU) of the hospital. The aim of this study was to evaluate the impact of the strengthened OHU of the hospital and what the current status of work-related accidents is like at the same hospital three years later.Methods: A cross-sectional study was conducted to investigate the  prevalence and context of all work-related accidents that resulted in contamination with blood and blood products inside the operating theatre, among surgical residents at the Tikur Anbessa teaching specialized referralhospital, in Addis Ababa University, Ethiopia. Data was collected from all 76 surgical residents who were at different stages of their specialty training in 2011.Results: Seventy- two (94.7%) of the residents were males and 26  (34.2%) were in their 3rd and 4th year of training. Of the 76 respondents, 53 (69.8%) had sustained a needle-stick injury inside the operating theatre at least once during their residency (Range=1-15 times). For 20 (26.3%), the accidents involved a high risk patient at least once. Cut with a sharp object, contact of blood to an unprotected skin and splash of blood to the eyes and face were reported by 9 (11.7%), 39(51.3%) and 28(36.9%) of the respondents respectively. Information concerning the most recent injuries inside the operating theatre revealed that 46(69.7%) of the residents sustained accidents in the 6 months preceding the survey, 7(9.2%) of which involved a high risk patient. All of the 7 (100%) of the recent high risk injuries and 10(27.7%) of the low-risk injuries were  reported to the OHU and all the high risk injury victims were commenced on HIV prophylaxis.Conclusions and Recommendations: Overall, the study revealed that work-related accidents among surgical trainees are still unacceptably high, even though there is a decline in the number of sharp object cut, and blood splash accidents. However, there is a positive trend towards reporting of injuries, particularly those which are high risk. More is expected from the hospital to create a safe working environment and to encourage reporting of all form of injuries

One-dimensional mechanistic model for flow boiling pressure drop in small- to micro- passages

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Accurate predictions of two-phase pressure drop in small to micro diameter passages are necessary for the design of compact and ultra-compact heat exchangers which find wide application in process and refrigeration industries and in cooling of electronics. A semi-mechanistic model of boiling two-phase pressure drop in the confined bubble regime is formulated, following the three-zone approach of Thome et al. (2004) for heat transfer. The total pressure drop is calculated by time-averaging the respective pressure drop values of single-phase liquid, elongated bubble with a thin liquid film and single-phase vapour. The model results were compared with experimental data collected for a wide range of diameter tubes (4.26, 2.88, 2.02, 1.1 and 0.52 mm) for R134a at 6 – 12 bar

Assessing the impacts of cassava technology on poverty reduction in Africa

In Africa, there have been successes in cassava research in terms of the development of production technologies, particularly improved varieties with high yield potential. The study addresses the question of whether and to what extent adoption of improved cassava varieties has led to rural poverty reduction in four African countries, namely Tanzania, Democratic Republic of Congo, Sierra Leone and Zambia. Data for the study come from a household survey conducted in the above-mentioned countries through a multinational-CGIAR support to agricultural research for development of strategic crops (SARD-SC) project in Africa. Given the observational nature of the data, a parametric approach (endogenous switching regression model) is applied. The results indicate that the model detects selectivity bias. Accounting for the bias, we find that adoption of cassava technology has resulted in an approximately 10 percentage point reduction in the poverty rate. Given an adoption rate of 34 per cent and a 10 percentage point reduction in the poverty rate, an estimated 24,309 households (equivalent to 194,469 individuals) have managed to move out of poverty in these four countries as a result of adoption of the technology. We also find that adoption of the technology has benefitted non-poor and female-headed households, relative to poor and male-headed households. The results present important evidence in favour of promoting cassava technology in a targeted fashion as part of an effective poverty reduction and sustained agricultural growth strategy in Africa. Considering the large realised and even more pronounced potential impacts of the adoption of cassava technology on poverty reduction, it is vital that regional and global development organisations should continue supporting the existing cassava improvement programme to sustain the technology development efforts in the continent