This study conducts an experimental study concerning the improvement of nozzle/diffuser micropump design using some novel no-moving-part valves. A total of three micropumps, including two enhancement structures having two-fin or obstacle structure and one conventional micro nozzle/diffuser design, are made and tested in this study. It is found that dramatic increase of the pressure drops across the designed micro nozzles/diffusers are seen when the obstacle or fin structure is added. The resultant maximum flow rates are 47.07 mm³/s and 53.39 mm³/s, respectively, for the conventional micro nozzle/diffuser and the added two-fin structure in micro nozzle/diffuser operated at a frequency of 400 Hz. Yet the mass flow rate for two-fin design surpasses that of conventional one when the frequency is below 425 Hz but the trend is reversed with a further increase of frequency. This is because the maximum efficiency ratio improvement for added two-fin is appreciably higher than the other design at a lower operating frequency. In the meantime, despite the efficiency ratio of the obstacle structure also reveals a similar trend as that of two-fin design, its significant pressure drop (flow resistance) had offset its superiority at low operating frequency, thereby leading to a lesser flow rate throughout the test range.

This study investigates the feasibility of using R-134a filled separated two-phase thermosiphon loop (STPTL) as a free cooling technique in datacenters. Two data center racks one of them is attached with fin and tube thermosiphon were cooled by CRAC unit (computer room air conditioning unit) individually. Thermosiphon can help to partially eliminate the compressor loading of the CRAC; thus, energy saving potential of thermosiphon loop was investigated. The condenser is a water-cooled design and perfluoroalkoxy pipes were used as adiabatic riser/downcomer for easier installation and mobile capability. Tests were conducted with filling ratio ranging from 0 to 90%. The test results indicate that the energy saving increases with the rise of filling ratio and an optimum energy savings of 38.7% can be achieved at filling ratios of 70%, a further increase of filling ratio leads to a reduction in energy saving. At a low filling ratio like 10%, the evaporator starves for refrigerant and a very uneven air temperature distribution occurring at the exit of data rack. The uneven temperature distribution is relieved considerably when the evaporator is fully flooded. It is also found that the energy saving is in line with the rise of system pressure. Overfilling of the evaporator may lead to a decline of system pressure. A lower thermal resistance occurs at high filling ratios and higher ambient temperature.

Two Si-based micro pulsating heat pipes (µPHPs) charged using HFE-7100 were either horizontally or vertically oriented and were tested using several heating powers. The width of each channel was 0.8 mm in one µPHP containing uniform channels, and the channel width was 1.0 mm or 0.6 mm in the other µPHP, which did not contain uniform channels. The depth of each channel was 0.25 mm. The overall size of each µPHP was 60 × 10 × 1.25 mm. Visual observation and temperature measurement of the µPHPs under various conditions were performed and the results were analyzed. The results indicated that when the µPHPs were operated horizontally at a heating power ranging from 1 to 7 W, the pulsating two-phase flow in the channels of the µPHPs could not begin, except when the µPHP containing nonuniform channels was tested at a heating power of 7 W. With a heating power less than 5 W, the frequency of the sine-like oscillating displacement of the vapor slug increased and the displacement of the vapor slug reduced in either vertically oriented μPHP, as the heating power increased With a heating power higher than 5 W, periodic “start-stop” behaviors were observed in the vertical μPHP containing nonuniform channels.

This study proposes a novel high-flux pulsating heat pipe that can lift the major constraint of the conventional pulsating heat pipe (PHP) which is unable to function properly upon anti-gravity operations. The proposed PHP introduces additional unbalance force via uneven tube diameter/geometry in the adiabatic sections to tailor the problem in anti-gravity operation. The design contains a three-dimensional configuration circuitry with compact arrangement tubes on the evaporator and condenser. Through this design, the non-uniform three-dimensional pulsating heat pipe (3D-PHP) manipulates the uneven inner diameters of the adiabatic sections to form uneven vapor/liquid distributions in the adiabatic sections to yield a unitary flow pattern that is able to withstand a much higher input power. The present PHP uses methanol as working fluid, with 38% volumetric filling ratio, and has a high-flux of 22.9 W/cm² and a low the thermal resistance ratio (R_anti-gravity/R_{gravity-assisted}) of 1.05 when the input power is 800 W. Both the heat flux and thermal resistance ratio for the proposed design are far better than the existing literature.

This study examines the performance of three heat pump dryers: the original reference design, a modified drying chamber, and an external desiccant wheel design. Unlike most existing studies that normally adopt organic products as the drying materials, in this study we used moist sodium polyacrylate (Orbeez) as the drying material for consistent characterization of the heat pump performance. R-134a was adopted as the refrigerant for the heat pump system. The experiments were performed subject to different weights of Orbeez (drying material) at a constant volumetric flow rate of 100 m³/h. During experimentation, different parameters like the coefficient of performance (COP_HP), drying rate, heat transfer rate by the condenser, moisture extraction rate, and specific moisture extraction rate were calculated. The average COP_HP, mass transfer rate, heat transfer rate, MER, and SMER of the system were calculated as 3.9, 0.30 kg/s, 0.56 kW, 0.495 kg/h, and 1.614 kg/kWh, respectively. The maximum COP for the refrigeration system was achieved at lower test loads with the desiccant wheel. The moisture extraction rate for a lower test loading was higher than that for a higher test load due to the higher penetration of drying air at the lower test load, although the maximum test load showed the maximum relative humidity at the dryer outlet. The desiccant wheel showed good performance in terms of moisture extraction rate and COP_HP, but it showed poor performance in terms of the specific moisture extraction rate due to the high power consumption (around 2.6 kW) of the desiccant dehumidifier. The moisture extraction rate (MER) for all designs increased to a maximum value, followed by consistent decline. However, the maximum MER for the desiccant design exceeded those for the other designs.