Flow Visualization of Two-Phase R-245fa at Low Mass Flux in a Plate Heat Exchanger near the Micro-Macroscale Transition

Two-phase R-245fa flow in a plate heat exchanger is experimentally investigated to understand the unique flow regimes found during operation at low refrigerant mass flux. A transparent plate heat exchanger replica with 3.4 mm hydraulic diameter is 3D-printed for flow visualization using high-speed videography. Observed flow regimes support that the thermofluidic characteristics peculiar to plate heat exchanger (PHE) operation are due to the macro-microscale transitional two-phase flow from the coexistence of fluid inertial force and surface tension effects, corresponding to the operation conditions. Maximum stable bubble diameter is bigger at low mass flux than at high mass flux, and the bubbles can become big enough to be fully confined in the millimeter-scale PHE channel to be deformed or elongated. This represents the main thermo-physical characteristics of two-phase flow in mini- and microchannels, which is different from turbulent mixing flow easily found at high-mass-flux operation or in channels of conventional macroscale. Flow morphology involving complex bubble coalescence and breakup dynamics is captured and analyzed in relation to the fluid properties and geometric obstructions provided by the plate heat exchanger channel. While there exist previous studies, and even heat transfer coefficient correlations, suggesting the potential microscale flow regimes in PHEs, this is the first time presenting evidences via flow visualization

Load-shifting opportunities for typical cement plants

Investigations into demand side reductions have been encouraged by the South African electricity utility, Eskom, in sectors with high electricity consumption, such as the cement industry. The South African cement industry is responsible for 5% of the electrical consumption for the mining and industrial sector. It has also been estimated that by 2020 this sector will be ranked fifth for energy savings potential. This paper investigates the potential of a load-shifting (altering energy use method) scheme to reduce evening peak loads and save electrical costs on a raw mill at a South African cement plant. A spread sheet-simulation was performed, which showed that six hours of load-shifting could be achieved, without adversely affecting production. This was corroborated by a pilot study where the load was successfully shifted for six hours over a week-long period. The specific raw mill would achieve a reduction in yearly electrical costs of 2% when employing this load-shifting strategy. The results, however, showed that cost-saving opportunities are highly dependent on the reliability of the mills and on the change in production demand. Therefore, load-shifting schemes have to be flexible on a daily basis to shift load whenever possible

A simple demand-side management solution for a typical compressed-air system at a South African gold mine

Once designed, mine compressed-air systems tend to operate at peak levels throughout the life of the mine, despite there being significant periods when this air quantity is not required. This is mainly due to lack of appropriate compressor controls. Consequently, such compressed-air systems are inefficient and wasteful. A compressed-air system at a South African gold mine was retrofitted with an automatic compressor control system featuring compressor cascading and pressure bandwidth control. The goal was to implement a simple demand-side management (DSM) strategy to afford meaningful electrical energy savings. The automatic control strategy realised a saving of 1.25 MW (on a baseline of 7.22 MW) during Eskom’s evening peak demand window. This represents a reduction of 17.3% in electrical power consumption during the evening peak period, and savings of nearly R2.9 million per year

Validation of in-tube condensation performance

Paper presented to the 1st International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Kruger Park, 8-10 July 2002.This paper reports on the condensation heat transfer and pressure drop performance of a heat pump water heater operating with R-22 at an average saturation temperature of 40°C with mass fluxes ranging from 300 - 800kg/m2s. The purpose of the experimental work was to validate the effectiveness of a computerized data acquisition system, as well as to establish a sound experimental procedure. Experimental flow regimes were determined using a well-known flow regime predictor. This showed that the experimental data points lay mainly in the annular flow region. The experimental heat transfer coefficients and pressure drops were compared with widely used correlations, and good agreement was obtained.tm201

A simple demand-side management solution for a typical compressed-air system at a South African gold mine

Once designed, mine compressed-air systems tend to operate at peak levels throughout the life of the mine, despite there being significant periods when this air quantity is not required. This is mainly due to lack of appropriate compressor controls. Consequently, such compressed-air systems are inefficient and wasteful. A compressed-air system at a South African gold mine was retrofitted with an automatic compressor control system featuring compressor cascading and pressure bandwidth control. The goal was to implement a simple demand-side management (DSM) strategy to afford meaningful electrical energy savings. The automatic control strategy realised a saving of 1.25 MW (on a baseline of 7.22 MW) during Eskom’s evening peak demand window. This represents a reduction of 17.3% in electrical power consumption during the evening peak period, and savings of nearly R2.9 million per year

Heat transfer and pressure drop characteristics of angled spiralling tape inserts in a heat exchanger annulus

Paper presented to the 1st International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Kruger Park, 8-10 April 2002.The purpose of this paper was to determine the single phase heat transfer and pressure drop characteristics of an angled spiralling tape inserted into the annulus of a tube-in-tube heat exchanger. Experimental measurements were taken on four setups: a normal tube-in-tube heat exchanger used as a reference and three heat exchangers with different angled spiralling tape inserts. From the results correlations were developed that can be used to predict the heat transfer and pressure drop characteristics. It was concluded that the angled spiralling tape inserts resulted in an increase in the heat transfer and pressure drop characteristics as can be expected.tm201

Development of a motion platform for an educational flight simulator

Flight simulators are regularly used in the undergraduate and postgraduate training of mechanical and aeronautical engineers. Due to advances in computing technology, several flight simulation-related tasks can now be accomplished in real-time using low-cost PC platforms and inexpensive commercial software. The difficulty in realising an educational flight simulator system with motion platform therefore lies with the design and construction of an effective motion platform. Costs become exorbitant when simulation platforms of more than two degrees of freedom (i.e. pitch and roll) are attempted. This paper describes the development of a drive system for a motion platform with two degrees of freedom (pitch and roll) for use in undergraduate engineering training. Use was made of off the-shelf PC equipment and flight simulation software and hardware, together with commercial actuators and drive systems. The motion platform was manufactured from square tubing and consisted of three frames: the stationary main frame and, rotating inside this, the roll frame and pitch frame. These rotated relative to each other and were actuated by two similar-sized DC motors and gearbox/ chain transmissions. The system effectively simulated the pitch and roll motions of commercial airliners, using a low-cost, easily maintainable motion platform. The educational value of the simulator was twofold: first, it was to be displayed in the science exploratorium (SciEnza) of the University of Pretoria; and second, it provided a platform on which mechanical (as well as electrical, electronic and computer) engineering students could conduct practical work in courses such as dynamics and control, and on which final-year and postgraduate students could conduct research.National Aerospace Centrehttp://www.manchesteruniversitypress.co.uk/cgi-bin/scribe?showinfo=ip017hb2016Mechanical and Aeronautical Engineerin

Influence of partial and complete glutamine-and glucose deprivation of breast-and cervical tumorigenic cell lines

BACKGROUND : Due to their high proliferative requirements, tumorigenic cells possess altered metabolic systems whereby cells utilize higher quantities of glutamine and glucose. These altered metabolic requirements make it of interest to investigate the effects of physiological non-tumorigenic concentrations of glucose and glutamine on tumorigenic cells since deprivation of either results in a canonical amino acid response in mammalian cell. METHODS : The influence of short-term exposure of tumorigenic cells to correlating decreasing glutamine- and glucose quantities were demonstrated in a highly glycolytic metastatic breast cell line and a cervical carcinoma cell line. Thereafter, cells were propagated in medium containing typical physiological concentrations of 1 mM glutamine and 6 mM glucose for 7 days. The effects on morphology were investigated by means of polarizationoptical transmitted light differential interference contrast. Flow cytometry was used to demonstrate the effects of glutamine-and glucose starvation on cell cycle progression and apoptosis induction. Fluorometrics were also conducted to investigate the effects on intrinsic apoptosis induction (mitocapture), reactive oxygen species production (2,7-dichlorofluorescein diacetate) and acidic vesicle formation (acridine orange). RESULTS : Morphological data suggests that glutamine-and glucose deprivation resulted in reduced cell density and rounded cells. Glutamine-and glucose starvation also resulted in an increase in the G2M phase and a sub-G1 peak. Complete starvation of glutamine and glucose resulted in the reduction of the mitochondrial membrane potential in both cell lines with MDA-MB-231 cells more prominently affected when compared to HeLa cells. Further, starved cells could not be rescued sufficiently by propagating since cells possessed an increase in reactive oxygen species, acidic compartments and vacuole formation. CONCLUSION : Starvation from glutamine and glucose for short periods resulted in decreased cell density, rounded cells and apoptosis induction by means of reactive oxygen species generation and mitochondrial dysfunction. In addition, the metastatic cell line reacted more prominently to glutamine-and glucose starvation due to their highly glycolytic nature. Satisfactory cellular rescue was not possible as cells demonstrated oxidative stress and depolarized mitochondrial membrane potential. This study contributes to the knowledge regarding the in vitro effects and signal transduction of glucose and/or L-glutamine deprivation in tumorigenic cell lines.Grants from the Cancer Association of South Africa, the Struwig Germeshuysen Trust, RESCOM (Research Council of the University of Pretoria), the South African National Research Foundation and Medical Research Council.http://www.cellandbioscience.comam201