MICRO BUBBLE FORMATION AND BUBBLE DISSOLUTION IN DOMESTIC WET CENTRAL HEATING SYSTEMS

16 % of the carbon dioxide emissions in the UK are known to originate from wet domestic central heating systems. Contemporary systems make use of very efficient boilers known as condensing boilers that could result in efficiencies in the 90-100% range. However, research and development into the phenomenon of micro bubbles in such systems has been practically non-existent. In fact, such systems normally incorporate a passive deaerator that is installed as a ‘default’ feature with no real knowledge as to the micro bubble characteristics and their effect on such systems. High saturation ratios are known to occur due to the widespread use of untreated tap water in such systems and due to the inevitable leakage of air into the closed loop circulation system during the daily thermal cycling. The high temperatures at the boiler wall result in super saturation conditions which consequently lead to micro bubble nucleation and detachment, leading to bubbly two phase flow. Experiments have been done on a test rig incorporating a typical 19 kW domestic gas fired boiler to determine the expected saturation ratios and bubble production and dissolution rates in such systems

The fundamentals of two-phase flow in wet domestic central heating systems

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.An emerging trend in the building services industry is the installation of passive deaerators on the flow line of domestic wet central heating systems. To date, no data and theoretical models predicting the two-phase flow characteristics in domestic wet central heating systems are available in the open literature. This gap in literature has prevented essential design improvements to passive deaerators thus impeding the efficiency enhancement of such devices. Hence, the current study is aimed at assisting designers of deaeration devices by providing fundamental data and model correlations with respect to the two-phase flow characteristics typical in a wet domestic central heating system. For this purpose an experimental research project was adopted and several studies were carried out, including; (1) a comprehensive review to understand the background of the phenomena, (2) the design and construction of an experimental test rig to conduct the necessary investigations into the phenomenon of two-phase flow in domestic wet central heating systems, (3) the development of a reliable image capture and analysis technique, (4) the completion of a number of experiments to investigate typical bubble sizes, volumetric void fractions, bubble distributions and nucleation and dissolution rates and (5) the correlation of the data gathered as part of the present study with existing bubble size, nucleation and dissolution prediction models. This research has, for the first time, provided an in depth analysis into two-phase flow characteristics in wet domestic central heating systems through the use of a high speed camera and image analysis techniques. The two-phase phenomenon finds its origins in high dissolved gas concentrations present in the water flowing through the closed loop system, thus resulting in super saturation conditions at the primary heat exchange wall conditions. Bubble sizes at the boiler flow line were found to be dependent on the bulk fluid velocity, heat flux and pressure, with a measured mean diameter in the range of 0.13 mm to 0.39 mm. The Winterton (1972a) force balance model for bubble size prediction was in reasonable agreement with the experimental results. This model was further improved through the correlation of our data with the inclusion of dimensionless groups. Bubble nucleation rates have been calculated in the range of 0.3 to 4 bubbles / cm2 s with total system bubble production rates measured in the range of 784 to 6920 bubbles per second. Bubble nucleation rates have been calculated through the consideration of the heat exchanger surface under super saturation conditions. A correlation for the model by Hepworth et al. (2003) for nonclassical heterogeneous nucleation is proposed based on the experimental data gathered during the present study. Experimental results have shown dissolution rates for the bubble size ratio in the range of 0.4 to 12 % per second with system conditions. A modification of the model developed by Epstein and Plesset (1950) for stationary bubble dissolution is proposed with the inclusion of the Sherwood number to capture the effects of turbulent diffusion. The volumetric void fraction distribution in vertical pipes was found to be quasi-homogenous across the pipe section while being strongly dependent on gravitational and turbulence effects in horizontal pipe bubbly flow. A CFD simulation predicted the volumetric void fraction distribution with reasonable accuracy.This study is funded by The Engineering and Physical Sciences Research Council, Spirotech bv. and STEPS Malta

CFD analysis of the two-phase bubbly flow characteristics in helically coiled rectangular and circular tube heat exchangers

Due to their ease of manufacture, high heat transfer efficiency and compact design, helically coiled heat exchangers are increasingly being adopted in a number of industries. The higher heat transfer efficiency over straight pipes is due to the secondary flow that develops as a result of the centrifugal force. In spite of the widespread use of helically coiled heat exchangers, and the presence of bubbly two-phase flow in a number of systems, very few studies have investigated the resultant flow characteristics. This paper will therefore present the results of CFD simulations for the two-phase bubbly flow in helically coiled heat exchangers as a function of the volumetric void fraction and the tube cross-section design. The CFD results are compared to the scarce flow visualisation experimental results available in the open literature

Numerical study on turbulent heat transfer and pressure drop characteristics of a helically coiled hybrid rectangular-circular tube heat exchanger with Al2O3-water nanofluids

In the present study, turbulent Al2O3-water nanofluid flow in helically coiled, hybrid rectangular-circular tubes under constant wall heat flux, was numerically investigated. The heat transfer and pressure drop characteristics were determined as a function of the nanoparticle volume concentration (1–4%) and coil curvature. The system parameters used are typical to those found in contemporary wet central heat systems, with Reynolds numbers and coil curvatures in the range of 10,000–60,000 and 0.032–0.052 respectively. The numerical computations were developed with the widely cited single-phase homogeneous model using ANSYS FLUENT. The model was validated against empirical correlations for the Nusselt number and friction factor of water in coiled tubes and against data for Al2O3-water nanofluid flow in a straight tube. Both the heat transfer coefficient and frictional pressure drop were enhanced with the nanoparticle concentration and curvature. A thermo-hydrodynamic performance index was used to appraise the overall impact, with results suggesting that the nanofluids will enhance the overall system performance. Correlations for the prediction of the Nusselt number and friction factor were fitted to our data with reasonable accuracy

The association between pleural fluid exposure and survival in pleural mesothelioma

Background: Most patients with malignant pleural mesothelioma (MPM) present with malignant pleural effusion (MPE). There is in vitro evidence that MPE may not be a simple bystander of malignancy, but potentially has biological properties improving cancer cell survival and promoting cancer progression. If this is the case, MPE management may need to shift from current symptomatic strategies to aggressive fluid removal to impact on survival. Research question: Is there an association between pleural fluid exposure and survival in MPM? Study design and Methods: Data on 761 patients diagnosed with MPM between 2008-2018 were collected from patient medical records in 3 UK pleural units. Data included factors previously identified as influencing prognosis in MPM. Medical imaging was reviewed for presence, size and duration of pleural effusion. Time-dependent covariate analysis of pleural fluid exposure and survival (model included weight loss, serum albumin, Hb, MPM subtype, performance status, chemotherapy, age), and multivariable cox regression analysis of pleurodesis and survival were conducted. Results: Median overall survival was 278 days (IQR 127-505, 95% CI 253-301). Pleural fluid exposure duration showed no association with survival (HR 1.0, 95% CI 1.0-1.0). Median survival was 473, 378 and 258 days with complete, partial, and no pleurodesis (p 0.008). Interpretation: Pleurodesis success appears to be associated with improved survival, however it is unclear whether duration of MPM exposure to pleural fluid is associated with survival within the limitations of this retrospective study. Future prospective studies are required to assess this potentially important mechanism

The STArgardt Remofuscin Treatment Trial (STARTT): Design and baseline characteristics of enrolled Stargardt patients

Background: This report describes the study design and baseline characteristics of patients with Stargardt disease (STGD1) enrolled in the STArgardt Remofuscin Treatment Trial (STARTT). Methods: In total, 87 patients with genetically confirmed STGD1 were randomized in a double-masked, placebo-controlled proof of concept trial to evaluate the safety and efficacy of 20 milligram oral remofuscin for 24 months. The primary outcome measure is change in mean quantitative autofluorescence value of an 8-segment ring centred on the fovea (qAF 8). Secondary efficacy variables are best corrected visual acuity (BCVA), low-luminance visual acuity (LLVA), mesopic microperimetry (mMP), spectral domain optical coherence tomography (SD-OCT), reading speed on Radner reading charts, and patient-reported visual function as assessed by the National Eye Institute Visual Functioning Questionnaire 25 (NEI VFQ-25) and Functional Reading Independence (FRI) Index. Results: Mean age of participants was 35±11 years with 49 (56%) female. Median qAF 8 value was 438 Units (range 210-729). Median BCVA and LLVA in decimal units were 0.50 (range 0.13-0.80) and 0.20 (range 0.06-0.63), respectively. The median of the mean retinal sensitivity with mMP was 20.4 dB (range 0.0-28.8). SD-OCT showed median central subfield retinal thickness of 142 m (range 72-265) and median macular volume of 1.65 mm 3 (range 1.13-2.19). Compared to persons without vision impairment, both reading performance and patient-reported visual function were significantly lower (p<0.001, one sample t-test). Mean reading speed was 108±39 words/minute with logRAD-score of 0.45±0.28. Mean VFQ-25 composite score was 72±13. Mean FRI Index score 2.8±0.6. Conclusions: This trial design may serve as reference for future clinical trials as it explores the utility of qAF 8 as primary outcome measure. The baseline data represent the largest, multi-national, STGD1 cohort to date that underwent standardized qAF imaging, reading speed assessment and vision-related quality of life measures which all contribute to the characterization of STGD1