Experimental Performance of a Membrane Desorber Operating under Simulated Warm Weather Condensation Temperatures

In absorption systems using the aqueous lithium bromide mixture, the Coefficient of Performance is affected by the desorber. The main function of this component is to separate the refrigerant fluid from the working mixture. In conventional boiling desorbers, constant heat flux and vacuum pressure conditions are necessary to carry out the desorption process, and usually, the absorbers are heavy and bulky; thus, they are not suitable in compact systems. In this study, a membrane desorber was evaluated, operating at atmospheric pressure conditions with a water/lithium bromide solution with a concentration of 49.6% w/w. The effects of the solution temperature, solution mass flow, and condensation temperature on the desorption rate were analyzed. The maximum desorption rate value was 6.1 kg/m2h with the following operation conditions: the solution temperature at 95.2 °C, the solution mass flow at 4.00 × 10−2 kg/s, and the cooling water temperature at 30.1 °C. On the other hand, the minimum value was 1.1 kg/m2h with the solution temperature at 80.2 °C, the solution mass flow at 2.50 × 10−2 kg/s, and the cooling water temperature at 45.1 °C. The thermal energy efficiency, defined as the ratio between the thermal energy used to evaporate the refrigerant fluid with respect to the total thermal energy entering the membrane desorber, varied from 0.08 to 0.30. According to the results, a high solution mass flow, a high solution temperature, and a low condensation temperature lead to an increase in the desorption rate; however, a low solution mass flow enhanced the thermal energy efficiency. The proposed membrane desorber could replace a conventional boiling desorber, especially in absorption cooling systems that operate at high condensation temperatures as in warm weather regions.

Impact of pretreatment on colour and texture of watermelon rind

Impact of pretreatment on colour and texture of watermelon rind The effect of osmotic dehydration pretreatment on water loss, solid gain, colour and textural change was investigated. Watermelon rind 1 x 1 cm size was immersed in sucrose solution of 40, 50 and 60° Brix after pretreatment with microwave and conventional boiling in water for 1, 3, and 5 min, respectively. Water loss and solid gain increased with the time of cooking and sugar concentration. Microwave pretreated samples showed higher water loss and solid gain. Increase in the time of cooking decreased the brightness of all the samples. Microwave pretreated samples showed higher ‘b’ values than conventionally pretreated ones. There was no significant difference (P≤0.05) in texture profile analysis parameters except for hardness. Hardness decreased with increase in time of cooking and sugar concentration. Second order regression model was developed for water loss and solid gain of microwave and conventional pretreated watermelon rind.

EXPERIMENTAL ANALYSIS OF POOL BOILING HEAT TRANSFER ON EXTENDED SURFACES AT NEAR VACUUM PRESSURES

This research paper presents a study of boiling heat transfer from longitudinal rectangular and square pin finned surfaces immersed in saturated water at low vapor pressures of 2 and 9 kPa. Conventional boiling analysis, which is based on the nominal surface area of the heater, was compared with a boiling analysis that considers the total "wetted" surface area.

Enhancement of Pool Boiling Heat Transfer Using Nanostructured Surfaces on Aluminum and Copper

System energy management and cooling for future advanced lasers, radars, and power electronics is gaining importance, resulting in an intense search for technologies and design techniques to dissipate ultra-high heat fluxes, reduce system energy usage, and increase system efficiencies. We report enhanced pool boiling critical heat fluxes (CHF) at reduced wall superheat on nanostructured Al and Cu substrates, which are commonly used in advanced electronics cooling applications. Nanostructured surfaces were realized by using a low temperature nanomaterial deposition process, Microreactor-assisted-nanomaterial-deposition (MAND™). Using this technique we deposited ZnO nano-structures on Al and Cu substrates. We varied a number of parameters such as micro-/nano-structure morphologies, pore sizes, densities, and their inter-connectivity to identify optimal morphologies. These surfaces displayed typical pore sizes of 50–100 nm with pore densities of about 100–200 per μm2 and hydrophilic/super-hydrophilic characteristics with measured contact angles as low as 0°. We have demonstrated the capability to control MAND™ processes to create static contact angle from about 58° to near 0° for ZnO on Al surfaces and 80° to about 40° for zeolite texturing on Si surfaces. Average roughness measured using Atomic Force Microscopy (AFM) was in the range of 200–600 nm. Pool boiling refers to boiling under natural convection and nucleate boiling conditions, where the heating surface is submerged in a large body of stagnant liquid and the relative motion of the vapor bubble and its surrounding liquid is primarily due to buoyancy effect. We have focused to date on water as test liquid, but have future plans for investigating HFE 7100. We observed pool boiling CHF of 80–82.5 W/cm2 for nanostructured ZnO on Al surfaces versus a CHF of 23.2 W/cm2 on a bare Al surface with a wall superheat reduction of 25–38 °C. These new CHF values on ZnO nanostructured surfaces correspond to a boiling heat transfer coefficient as high as ∼23000 W/m2K. This represents an increase of almost 4X in CHF on nano-textured surfaces, which is contrary to conventional boiling heat transfer theory. We will discuss our current data, compare the behavior with conventional boiling theory, and compare with similar recent boiling heat transfer data on other nano-scale surfaces by other researchers. We are currently investigating these surfaces under forced convection conditions in microchannels to assess their flow boiling capabilities.

A Computer-Aided Molecular Design of Fluids That Optimize Absorption Cycle COP

A method to generate new absorbents for water in a novel refrigeration cycle was developed. The cycle, known as the phase separation absorption cycle, relies on phase separation, rather than boiling, to generate the refrigerant. After separation, the refrigerant flows to a conventional evaporator, where it evaporates, resulting in a cooling effect. The vapor is absorbed in a conventional absorber. The phase separation process is projected to require less energy than the conventional boiling process, resulting in higher cycle performance. However, suitable fluids for this cycle have never been found. The computer program developed under this work was used for the systematic design of possible absorbents. Whereas the predictive techniques which serve as kernel of the program can only provide approximate results, they allow fast screening of candidate absorbents. A model of the cycle was run for each potential absorbent, to establish whether the cycle and absorbent were compatible. A total of over 250,000 molecules were investigated, allowing the identification of a few with the potential to work in conjunction with the cycle.

Performance of Some Heat-Sensitive Differential Agars Prepared and Melted by Microwave Energy1

The viable cell count performance of some heat-sensitive differential agars prepared and remelted by microwave energy was evaluated for Salmonella choleraesui, Streptococcus faecalis and Escherichia coli. The conventional boiling method was used for comparison. No significant difference was found between the microwave oven processed agar and the conventional-boiling processed agar in viable cell counts of the target bacteria. Heating and reheating of violet red bile agar, bismuth sulfite agar, and KF Streptococcus agar by both methods did not change agar performance. However, remelting of desoxycholate citrate agar by both methods resulted in a substantial lowering of viable cell counts.

Melting Agar by Microwave Energy

The microwave oven is very convenient for melting agar for viable cell counts. Composite data of four microwave ovens indicated that melting time for 50 ml of agar per bottle was about 1 min for one bottle, 1.5 min for two bottles, and 2.5 min for four bottles heated simultaneously. Melting time for 100 ml of agar per bottle was about 1.5 min for one bottle, 2.5 min for two bottles, and 4 min for four bottles. Melting times of agar in square or flat bottles were similar. Agar melted by microwave treatment performed in viable cell counts equally as well as agar melted by the conventional boiling method. Even after prolonged (50% longer than melting time) microwave treatment, performance of the agar remained unchanged. Agar melted by microwave treatment can remain in liquid form (48°C) in situ for about 30 min (50 ml) and 1 h (100 ml). When removed from the microwave oven immediately after melting, the agar remained in liquid form (48°C) at room temperature for about 25 min (50 ml) and 40 min (100 ml). The microwave oven is highly efficient in melting agar without detrimental effects on the performance of agar.