Heat exchanger design effect on the system performance of silica gel adsorption refrigeration systems

This article presents a numerical investigation of the heat exchanger design effect on the performance of closed cycle, two-bed adsorption cooling systems with silica gel as adsorbent and water as refrigerant. It is well known that the shorter the cycle time, lower is the performance (cooling capacity and coefficient of performance). A long cycle time is responsible for lower cooling capacity. In this study, a non-dimensional switching frequency, which is inversely proportional to the cycle time, is defined and an optimum switching frequency is derived based on parametric analysis. The effect of other heat exchanger design parameters such as adsorbent number of transfer unit (NTU), bed Biot number (Bi), the heat exchanger aspect ratio (Ar) and the ratio of fluid channel radius to the adsorbent thickness (Hr), on the system performance has been investigated. The results show that the switching frequency ω, bed NTU, Ar and bed Bi have strong effects on the system performance. It is also seen that for a given set of design parameters, the system has an optimum switching frequency and the system performance will be declined seriously if the system is not operated at optimum switching frequency. The optimum switching frequency increases with the increase of NTU, Hr and with the decrease of Bi and Ar.     

Tungsten oxide nanowires grown on carbon paper as Pt electrocatalyst support for high performance proton exchange membrane fuel cells

Tungsten oxide nanowires (W₁₈O₄₉ NWs) were directly grown on carbon paper by chemical vapor deposition. Well-dispersed Pt nanoparticles, with a size distribution from 2 to 4 nm, were deposited on the surface of W₁₈O₄₉ NWs through a simple reductive process. The resulting Pt/W₁₈O₄₉ NW/carbon paper composites formed a three-dimensional electrode structure. In comparison to conventional Pt/C electrocatalyst, the Pt/W₁₈O₄₉ NW/carbon paper composite exhibited higher electrocatalytic activity toward the oxygen reduction reaction and better CO tolerance in a single cell polymer electrolyte membrane fuel cell.     

Facile Fabrication of Nanostructured Microchannels for Flow Boiling Heat Transfer Enhancement

Flow boiling in microchannels promises high heat transfer due to the combined effect of latent heat of vaporization and forced convection in confined spaces. However, flow boiling based miniaturized thermal management devices are limited due to instability induced dryout. While several efforts have been made to delay instabilities via advanced surface modification techniques, there is a need to expand the scope of applications by developing low-cost and scalable fabrication technologies for commonly used heat exchanger materials. In this paper, we use a facile and self-limiting chemical oxidation technique for fabricating sharp needle-like superhydrophilic CuO nanostructures within six parallel 500 × 250 µm² microchannels spread uniformly over a 1 × 1 cm² area in a copper heat sink. We demonstrate heat transfer enhancement with nanostructured microchannels (NSM) without any appreciable change either in the average pressure drop or the fluctuations in comparison to baseline plain wall microchannels (PWM). Analysis of the high-speed images was performed to attribute the enhancement with NSM to the presence of a capillarity-fed thin-film evaporation regime, which otherwise was absent in PWM. We believe that these results are encouraging and suggest that the heat sink geometry can be optimized to investigate the true potential of nanostructured microchannels.     

Widegap a-Si:H films prepared at low substrate temperature

Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E_g ∼ 1.90 eV and σ_ph ≥ 10⁻⁴ Scm⁻¹ has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.     

Study on an advanced adsorption desalination cycle with evaporator–condenser heat recovery circuit

This paper presents the results of an investigation on the efficacy of a silica gel–water based advanced adsorption desalination (AD) cycle with internal heat recovery between the condenser and the evaporator. A mathematical model of the AD cycle was developed and the performance data were compared with the experimental results. The advanced AD cycle is able to produce the specific daily water production (SDWP) of 9.24 m³/tonne of silica gel per day at 70 °C hot water inlet temperature while the corresponding performance ratio (PR) is comparatively high at 0.77. It is found that the cycle can be operational at 50 °C hot water temperature with SDWP 4.3. The SDWP of the advanced cycle is almost twice that of the conventional AD cycle.     

Hungry for tobacco: an analysis of the economic impact of tobacco consumption on the poor in Bangladesh

OBJECTIVE: To investigate the extent of tobacco expenditures in Bangladesh and to compare those costs with potential investment in food and other essential items. DESIGN: Review of available statistics and calculations based thereon. RESULTS: Expenditure on tobacco, particularly cigarettes, represents a major burden for impoverished Bangladeshis. The poorest (household income of less than $24/month) are twice as likely to smoke as the wealthiest (household income of more than $118/month). Average male cigarette smokers spend more than twice as much on cigarettes as per capita expenditure on clothing, housing, health and education combined. The typical poor smoker could easily add over 500 calories to the diet of one or two children with his or her daily tobacco expenditure. An estimated 10.5 million people currently malnourished could have an adequate diet if money on tobacco were spent on food instead. The lives of 350 children could be saved each day. CONCLUSION: Tobacco expenditures exacerbate the effects of poverty and cause significant deterioration in living standards among the poor. This aspect of tobacco use has been largely neglected by those working in poverty and tobacco control. Strong tobacco control measures could have immediate impact on the health of the poor by decreasing tobacco expenditures and thus significantly increasing the resources of the poor. Addressing the issue of tobacco and poverty together could make tobacco control a higher priority for poor countries.