Hassan Ali

Effects of Vapor Velocity and Pressure on Marangoni Condensation of Steam-Ethanol Mixtures on a Horizontal Tube

Careful heat-transfer measurements have been conducted for condensation of steamethanol mixtures flowing vertically downward over a horizontal, water-cooled tube at pressures ranging from around atmospheric down to 14kPa. Care was taken to avoid error due to the presence of air in the vapor. The surface temperature was accurately measured by embedded thermocouples. The maximum vapor velocity obtainable was limited by the maximum electrical power input to the boiler. At atmospheric pressure this was 7.5m/s while at the lowest pressure a velocity of 15.0m/s could be achieved. Concentrations of ethanol by mass in the boiler when cold prior to start up were 0.025%, 0.05%, 0.1%, 0.5%, and 1.0%. Tests were conducted for a range of coolant flow rates. Enhancement of the heat-transfer coefficient over pure steam values was found by a factor up to around 5, showing that the decrease in thermal resistance of the condensate due to Marangoni condensation outweighed diffusion resistance in the vapor. The best performing compositions (in the liquid when cold) depended on vapor velocity but were in the range 0.025‐0.1% ethanol in all cases. For the atmospheric pressure tests the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 15K, increased from around 55kW/m 2 K to around 110kW/m 2 K as the vapor velocity increased from around 0.8 to 7.5m/s. For a pressure of 14kPa the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 9K, increased from around 70kW/m 2 K to around 90kW/m 2 K as the vapor velocity increased from around 5.0 to 15.0m/s. Photographs showing the appearance of Marangoni condensation on the tube surface under different conditions are included in the paper. [DOI: 10.1115/1.4007893]

EFFECT OF CONDENSATE FLOW RATE ON RETENTION ANGLE ON HORIZONTAL LOW-FINNED TUBES

The paper reports experimental results using simulated condensation on eight horizontal integral finned tubes with different fin spacing but same root diameter. Condensation was simulated with low approaching zero vapor velocity of condensate using three liquids (water, ethylene glycol and R141b) supplied to the tube via small holes between the fins along the top of the tubes. Controlling parameters of the investigation were fin spacing of condensation tubes, flow rate of condensate and surface tension to density ratio of the condensate. The results indicate that the retention angle (measured from the top of the tube to the position where the inter-fin space is completely filled with liquid) increases with the increase in fin spacing. Also, retention angle increases as the density of the condensate increases but retention angle decreases with increase in surface tension. Interesting finding is seen as retention angle remains constant with increase in condensate flow rate, starting from very low (nearly zero) flow rate to the flow rate at which the tube gets fully flooded. The critical flow rate for eight tubes of defined fin density against three working fluids is measured.xa0 Results obtained from simulated condensation for almost zero condensate velocity are in good agreement with earlier data and theoretical model for retention angle on such tubes [1].

PERFORMANCE INVESTIGATION OF PHOTOVOLTAIC MODULES BY BACK SURFACE WATER COOLING

The temperature of the photovoltaic module has an adverse effect on the performance of photovoltaic modules. The photovoltaic module converts a small portion of energy from solar radiations into electricity while the remaining energy wastes in the form of heat. In this study, water cooled photovoltaic/thermal system was analyzed to enhance the efficiency by absorbing the heat generated by the photovoltaic modules and allowing the photovoltaic module to work at comparatively low temperature. For this system, four photovoltaic modules of two different types were used. To investigate the cooling effect, two modules were modified by making ducts at their back surface having inlet and outlet manifolds for water-flow. The measurements were taken with cooling and without cooling of photovoltaic modules. The temperature was measured at inlet, outlet, and at different points at the back of photovoltaic modules. It was found that there was a linear trend between the module efficiency and temperature. The average module temperature of c-Si and p-Si modules without cooling was 13.6% and 7.2% lower, respectively, than the same modules without cooling. As a result of temperature drop, the average module electrical efficiency of c-Si and p-Si was 13% and 6.2% higher, respectively, compared to the modules without cooling. Flowing water also gains useful heat from photovoltaic module so the resultant overall energy of the system was much higher.

Nonwovens modelling: a review of finite-element strategies

This paper reviews the main strategies used to simulate the mechanical behaviour of nonwoven materials that is defined by a structure of their fibrous networks and a mechanical behaviour of constituent fibres or filaments. The main parameters influencing the network structure of nonwoven materials are discussed in the first part. The second part deals with two main strategies employed in the analysis of mechanical behaviour of nonwoven materials using finite-element models based on continuous and discontinuous techniques. Both strategies have further subtypes, which are critically reviewed, and future trends in this area of research are discussed.

Enhanced Condensation of Ethylene Glycol on Three-Dimensional Pin-Fin Tubes

New experimental data are reported for condensation of ethylene glycol at near atmospheric pressure and low velocity on six three-dimensional pin-fin tubes. Enhancements of the vapour-side, heat-transfer coefficients were found between 3 to 5.5 when compared to a plain tube at the same vapour-side temperature difference. Heat-transfer enhancement was found to be strongly dependent on the active surface area i.e. on the proportion of the tube and pin surface not covered by condensate retained by surface tension. For all the tubes, vapour-side, heat-transfer enhancements were found to be approximately 3 times the corresponding active-area enhancements. The best performing pin-fin tube gave a heat-transfer enhancement of up to 5.5; 17% higher than those obtained from ‘optimised’ two-dimensional fin-tubes reported in the literature and about 24% higher than the ‘equivalent’ two-dimensional integral-fin tube (i.e. with same fin root diameter, longitudinal fin spacing and thickness and fin height).Copyright

Key design features of multi vacuum glazing for windows: A review

The use of vacuum glazed windows is increasing due to their application in mod-ern building design. Among various types of vacuum glazed windows reported in literature, thermal transmittance of single glass sheet (conventional window) i. e 6 W/m2k is reduced by 66 and 77% using air filled double glazed and air filled triple glazed windows, respectively. Using low emittance coatings thermal transmittance of double glazed windows is reduced by 53%, however it offsets the visibility by reduc-ing light transmittance by 5%. Stresses due to temperature/pressure gradients if not eliminated may lead to reduction in service life of vacuum glazed windows. Vacuum created between the glass sheets is used to reduce conductive heat transfer. Degrada-tion in the vacuum is caused by number of factors such as, permeation of gaseous molecules through glass sheets, leakage through sealing, thermal/optical desorption, and photo-fragmentation of organic species have been critically reviewed and future trends are outlined.

Drilling resistance: A method to investigate bone quality

PURPOSEnBone drilling is a major part of orthopaedic surgery performed during the internal fixation of fractured bones. At present, information related to drilling force, drilling torque, rate of drill-bit penetration and drill-bit rotational speed is not available to orthopaedic surgeons, clinicians and researchers as bone drilling is performed manually.nnnMETHODSnThis study demonstrates that bone drilling force data if recorded in-vivo, during the repair of bone fractures, can provide information about the quality of the bone. To understand the variability and anisotropic behaviour of cortical bone tissue, specimens cut from three anatomic positions of pig and bovine were investigated at the same drilling speed and feed rate.nnnRESULTSnThe experimental results showed that the drilling force does not only vary from one animal bone to another, but also vary within the same bone due to its changing microstructure. Drilling force does not give a direct indication of bone quality; therefore it has been correlated with screw pull-out force to provide a realistic estimation of the bone quality. A significantly high value of correlation (r2 = 0.93 for pig bones and r2 = 0.88 for bovine bones) between maximum drilling force and normalised screw pull-out strength was found.nnnCONCLUSIONSnThe results show that drilling data can be used to indicate bone quality during orthopaedic surgery.