Turbulent Heat Transfer of Supercritical Carbon Dioxide in the Proximity of the Pseudo-Critical Point with Non-Uniform Heat Flux Boundary Conditions

Seven decades of supercritical heat transfer research have not seen an attempt to characterize turbulent thermal transport in internal flows subject to asymmetric heating. Existing heat transfer models therefore do not account for phenomena that dictates thermal transport, in the proximity of the pseudo-critical point, for non-uniform heating. Additionally, past investigations almost exclusively employed macroscale circular test sections and no systematic investigations were conducted to assess the influence of channel geometry on supercritical heat transfer. This dissertation addressed these shortcomings. Thermal-hydraulic performance of supercritical carbon dioxide was characterized in non-circular microscale geometries ( 525.2 ≤ D_h ≤ 923 µm) for a wide range of experimental variables, including heat flux (5 ≤ q''≤50 W cm-2), mass flux (315 ≤G ≤2000 kg m- 2 s -1), and reduced pressure (1.03 ≤PR ≤1.1). In the two phases of the current investigation, the degree of asymmetry in applied heating was varied. In phase---I experiments, three non-uniformly heated stainless steel test sections, with different channel geometries, were employed. A data analysis technique, employing 2-D and 3-D heat transfer models of the test section, was developed to calculate the average heat transfer coefficients. Limiting the applied heating to the bottom wall of a microchannel based test section was the chief aim for phase---II experiments. This was achieved by using a combination of materials, Torlon 4203, Inconel-718 and stainless steel to build the test section. Infrared thermography was used to obtain local heat transfer data. For non-uniformly heated stainless steel test sections, using micro-pin based geometry and increasing channel aspect ratio led to higher heat transfer coefficients but with a commensurate penalty in pressure drop. Some existing heat transfer correlations were able to predict the heat transfer data from phase---I experiments with a MAPE of under 46%. However, when the applied heating was limited to the bottom wall, in phase---II, existing correlations proved to be inadequate. To address this, a reduced order predictive heat transfer model was developed. The model accounted for variable thermophysical property variations, flow acceleration, and unstable stratification effects. This model was able to predict 87.5% of the experimental data with a MAPE of less than 20 %.The tools and methodology developed in this dissertation can be used to design devices that will employ supercritical working fluids and will be subject to non-uniform heating

Experimental investigation of supercritical carbon dioxide in horizontal microchannels with non-uniform heat flux boundary conditions

Supercritical carbon dioxide (sCO2) experiences a drastic change in its thermophysical properties near the thermodynamic critical point. A non-linear thermophysical property variation can influence the heat transfer behavior of sCO2 which is not predicted well by conventional single phase heat transfer theory. This can become a major hindrance in the effective design of heat exchangers using sCO2 as a heat transfer fluid and operating in the vicinity of the critical point. Previous investigations of sCO2 heating have been primarily focused on macroscale, circular and uniformly heated channels at relatively low heat fluxes. It is unclear if models and correlations developed from large circular tube data can be scaled down to the microscale, non-circular channels subject to non-uniform heating. The present study experimentally investigates the turbulent heat transfer performance of sCO2 in a microchannel heat exchanger operating in a horizontal configuration with a single wall non-uniform heat flux boundary condition. The test section has five parallel channels with a 0.75 mm hydraulic diameter and an aspect ratio of 1. The channels are fabricated using computer numerical control machining and the test section sealed using a diffusion bonding approach. Data analysis techniques which employ 2-D and 3-D heat transfer models of the experimental test section are developed to calculate the average heat transfer coefficients for a given set of experimental conditions. Data are obtained over a wide range of experimental parameters including test section applied heat flux, mass flux , reduced pressure, and inlet temperatures. The heat transfer data were screened for the presence of buoyancy and flow acceleration effects and then compared against correlations developed for turbulent subcritical and supercritical fluid flows

Geometry and orientation effects in non-uniformly heated microchannel heat exchangers using supercritical carbon dioxide

There is growing interest to use supercritical carbon dioxide (sCO2) as a working fluid in thermal management applications. This study investigates the thermal-hydraulic performance of microchannel heat sinks as a function of flow channel geometry and orientation at operating conditions representative of electronics cooling applications. Three different experimental test sections, subject to non-uniform heat flux boundary conditions, were investigated. Two of the test sections contained parallel arrays of rectangular microchannels with a hydraulic diameter of 750 m and aspect ratios of 1 and 2, respectively. The third test section had a staggered array of diamond shaped micro-pins with a hydraulic diameter, based on the minimum flow area, of 525.2 m. Data were collected for varying inlet temperature, mass flux, and heat flux at a fixed reduced pressure of 1.1. A data analysis method using 2-D and 3-D heat transfer models of the test sections was used to calculate the average heat transfer coefficients for each experimental condition. Additionally, a pressure drop model was developed to resolve the total measured pressure drop into its components. The results of this study indicate that the turbulent convective heat transfer was independent of orientation (top versus bottom heating) for square microchannel (aspect ratio = 1) for the conditions investigated. Increasing the channel aspect ratio from 1 to 2 led to an enhancement in thermal transport. Finally, the heat transfer performance of the staggered pin array flow geometry was superior to the rectangular channels, but this enhancement in heat transfer was commensurate with the increase in pressure drop. Based on these results, this paper concludes with general design recommendations for those considering the early adoption of supercritical carbon dioxide for thermal management applications

“Sex without fear”: exploring the psychosocial impact of oral HIV pre-exposure prophylaxis on gay men in England

Gay, bisexual, and other men who have sex with men (GBMSM) experience a high prevalence of psychosocial health problems, such as harmful substance use and depression, as well as being disproportionately affected by HIV. HIV Pre-Exposure Prophylaxis (PrEP) may provide psychosocial benefits beyond its intended purpose of reducing HIV infection. We explore the psychosocial impact of oral PrEP use on gay men in England using qualitative data from the PROUD study. From February 2014 to January 2016, semi-structured in-depth interviews were conducted with 40 gay men and one trans woman. Participants were purposively recruited based on trial arm allocation, adherence, and sexual risk behaviours. By removing HIV risk from sex, PrEP improves users’ wellbeing by reducing HIV-related anxiety and internalised stigma and increasing HIV prevention self-efficacy, sexual pleasure, and intimacy. In turn, these psychological changes may influence behaviour in the form of greater sexual freedom, reduced harmful drug use, and more protective sexual health behaviours. However, PrEP may create internal conflict for some gay men, due to its disruption of social norms around condom use and its perceived influence on their sexual behaviour leading to reduced condom self-efficacy. These findings provide a baseline of PrEP’s psychosocial impact amongst some of the first PrEP users in England and supports calls to consider the psychosocial impact of PrEP in prescribing guidelines

“Sex without fear”: exploring the psychosocial impact of oral HIV pre-exposure prophylaxis on gay men in England

Gay, bisexual, and other men who have sex with men (GBMSM) experience a high prevalence of psychosocial health problems, such as harmful substance use and depression, as well as being disproportionately affected by HIV. HIV Pre-Exposure Prophylaxis (PrEP) may provide psychosocial benefits beyond its intended purpose of reducing HIV infection. We explore the psychosocial impact of oral PrEP use on gay men in England using qualitative data from the PROUD study. From February 2014 to January 2016, semi-structured in-depth interviews were conducted with 40 gay men and one trans woman. Participants were purposively recruited based on trial arm allocation, adherence, and sexual risk behaviours. By removing HIV risk from sex, PrEP improves users’ wellbeing by reducing HIV-related anxiety and internalised stigma and increasing HIV prevention self-efficacy, sexual pleasure, and intimacy. In turn, these psychological changes may influence behaviour in the form of greater sexual freedom, reduced harmful drug use, and more protective sexual health behaviours. However, PrEP may create internal conflict for some gay men, due to its disruption of social norms around condom use and its perceived influence on their sexual behaviour leading to reduced condom self-efficacy. These findings provide a baseline of PrEP’s psychosocial impact amongst some of the first PrEP users in England and supports calls to consider the psychosocial impact of PrEP in prescribing guidelines

Local iontophoretic administration of cytotoxic therapies to solid tumors

Parenteral and oral routes have been the traditional methods of administering cytotoxic agents to cancer patients. Unfortunately, the maximum potential effect of these cytotoxic agents has been limited because of systemic toxicity and poor tumor perfusion. In an attempt to improve the efficacy of cytotoxic agents while mitigating their side effects, we have developed modalities for the localized iontophoretic delivery of cytotoxic agents. These iontophoretic devices were designed to be implanted proximal to the tumor with external control of power and drug flow. Three distinct orthotopic mouse models of cancer and a canine model were evaluated for device efficacy and toxicity. Orthotopic patient-derived pancreatic cancer xenografts treated biweekly with gemcitabine via the device for 7 weeks experienced a mean log2 fold change in tumor volume of −0.8 compared to a mean log2 fold change in tumor volume of 1.1 for intravenous (IV) gemcitabine, 3.0 for IV saline, and 2.6 for device saline groups. The weekly coadministration of systemic cisplatin therapy and transdermal device cisplatin therapy significantly increased tumor growth inhibition and doubled the survival in two aggressive orthotopic models of breast cancer. The addition of radiotherapy to this treatment further extended survival. Device delivery of gemcitabine in dogs resulted in more than 7-fold difference in local drug concentrations and 25-fold lower systemic drug levels than the IV treatment. Overall, these devices have potential paradigm shifting implications for the treatment of pancreatic, breast, and other solid tumors

Beetroot supplementation lowers daily systolic blood pressure in older, overweight subjects

Although inorganic nitrate and beetroot juice supplementation are associated with decreased systolic blood pressure (BP), these results have primarily been obtained from short-term trials that focused on healthy young adults. Therefore, we hypothesized that oral supplementation of beetroot juice concentrate would decrease systolic BP in overweight older participants but that the decline in BP would not be sustained after a 1-week interruption of the beetroot juice supplementation. For 3 weeks, 24 participants were randomized to either the beetroot juice concentrate or blackcurrant juice group, with a 1-week postsupplementation phase (week 4). Changes in systolic and diastolic BP were assessed during the supplementation and postsupplementation phases. Blood pressure was measured using 3 different methods: (1) resting clinic BP, (2) 24-hour ambulatory BP monitoring, and (3) home monitoring of daily resting BP. The first 2 methods were applied at baseline and after weeks 3 and 4. Daily measurements were conducted throughout the study, with 21 subjects completing the study (beetroot/blackcurrant = 10/11; male/female = 12/9; age = 62.0 ± 1.4 years; body mass index = 30.1 ± 1.2 kg/m2). After 3 weeks, beetroot juice supplementation was not associated with significant changes in resting clinic BP or 24-hour ABPM. Conversely, beetroot juice concentrate reduced daily systolic BP after 3 weeks (−7.3 ± 5.9 mm Hg, P = .02); however, the effect was not maintained after the interruption of the supplementation (week 4, 2.8 ± 6.1 mm Hg, P = .09). In overweight older subjects, beetroot juice concentrate supplementation was associated with beneficial effects on daily systolic BP, although the effects were not significant when measured by 24-hour ABPM or resting clinic BPPeer reviewe