Wibawa Endra Juwana

Bio: Wibawa Endra Juwana is an academic researcher from Sebelas Maret University. The author has contributed to research in topics: Bubble & Reynolds number. The author has an hindex of 6, co-authored 32 publications receiving 99 citations. Previous affiliations of Wibawa Endra Juwana include Diponegoro University & UniSource Energy Corporation.

Hydrodynamic characteristics of the microbubble dissolution in liquid using orifice type microbubble generator

Abstract: The aim of this work was to investigate the characteristics of oxygen dissolution into water in aeration using an orifice type microbubble generator (MBG). The analysis was conducted using dimensional analysis by taking into account bubble diameter distribution and oxygen mass transfer rate. The characteristics of microbubbles produced by MBG were affected by the combination of air and water flow rates through the MBG. In this study, the ranges of air and water flow rates were set at 0.1–1.0 l/min and 30.0–80.0 l/min, respectively. The microbubbles were captured by a high-speed camera using shadow photographic technique to obtain the bubbles size distribution as well as the average diameter of the bubbles. The volumetric mass transfer coefficient of oxygen was measured using dynamic physical absorption model. The results verified that the size of microbubble depend on air and water flow rates. The increase of the gas flow rate increased the average bubble diameter. On the other hand, the increase of the water flow rate decreased the average bubble diameter. Furthermore, the increase of water flow rate increased the oxygen volumetric mass transfer coefficient. Finally, by using dimensional analysis, the empirical correlation of volumetric mass transfer coefficient, average bubble diameter, and bubble diameter distribution, was proposed. The correlation showed that liquid Reynolds number played most important role in average bubble diameter and oxygen mass transfer rate obtained in MBG aeration.

The Effect of Heat Sink Properties on Solar Cell Cooling Systems

Abstract: High energy demand is leading to the replacement of fossil energy with renewable sources such as solar energy. Solar cells are devices used to generate solar energy. However, when exposed to sunlight with high intensity, a solar cell can suffer a decrease in performance due to overheating. This issue can be addressing by adding a cooling system. This study used a passive cooling system by adding a heat sink with fins to the body panel of the solar cell. The advantage of the passive cooling system is that it does not require additional energy. The number of fins and types of material were varied to obtain the best passive cooling system. The number of fins used was 5, 10, and 15, and the materials used were aluminum and copper. The wind speed vector and the temperature distribution were investigated through simulation. The results showed an increase in the number of fins provided better cooling capacity and increased the photovoltaic performance. The best cooling capability and performance were obtained using 15 fins with a copper base and fin heat sink materials. The decrease in temperature and the increase in efficiency were 10.2 °C and 2.74%, respectively. Therefore, the use of passive cooling system based on heat sinks with fins could provide a potential solution to increase performance and prevent overheating of photovoltaic (PV) panel systems.

Principles Of Enhanced Heat Transfer

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Internal Flow in an Enhanced Tube Having Square-cut Twisted Tape Insert

Abstract: In this study, a numerical simulation has been conducted in order to evaluate the thermal hydraulic performance of a turbulent single-phase flow inside an enhanced tube equipped with a square-cut twisted tape (STT) insert. The classical twisted tape (CTT) insert was also investigated for comparison. The k-e renormalized group turbulence model has been utilized as the turbulent model. Various twist ratios (y/W) of 2.7, 4.5, and 6.5 were investigated for the Reynolds number range of 8000–18,000, with water as the working fluid. The numerical results indicated that, in comparison with the plain tube (PT), the tube equipped with the STT with the twist ratios of 2.7, 4.5, and 6.5 led to an increase in the values of the Nusselt number and friction factor in the inner tube by 45.4–80.7% and 2.0–3.3 times, respectively; in addition, the highest thermal performance of 1.23 has been obtained. The results further indicated that the tube equipped with the CTT of the same twist ratios improved the Nusselt number and friction factor in the inner tube by 40.3–74.4% and 1.7–3.0 times, respectively, in comparison with the PT; further, the maximum thermal performance of 1.18 was achieved.

A PIV investigation of stratified gas–liquid flow in a horizontal pipe

Abstract: Abstract Simultaneous Particle Image Velocimetry (PIV) measurements of stratified turbulent air/water flow in a horizontal pipe have been performed using small water droplets, d p ‾ = 6 μ m , as tracer particles in the gas-phase. This seeding technique ensures that the surface tension of the water layer remains unaffected upon contact with the tracer particles in the gas-phase and thus allows small scale interfacial structures, such as capillary waves to occur and evolve naturally. Experiments have been conducted in a 31 m long, 100 mm in diameter PVC pipe using air and water at atmospheric pressure as test fluids. For the purpose of validation of the experimental set-up and the suggested seeding technique, gas single-phase measurements were performed at Re D = 45 , 000 and compared to existing DNS results from the literature with similar Re-number, showing very good agreement. Two stratified flow cases, i.e. smooth and wavy, are extensively discussed with emphasis on the effect of the interface pattern on the gas streamwise turbulence profile u g ′ . A simple analysis using the u g ′ -profiles of 17 stratified flows suggests the presence of a correlation between the turbulence structure of the gas-phase and global flow conditions such as the pressure drop and the bulk velocity.

Experimental investigation on the thermal performance of compact heat exchanger and the rheological properties of low concentration mono and hybrid nanofluids containing Al2O3 and CuO nanoparticles

Abstract: The present work focused on preparing and studying the fluid and thermal properties such as density, viscosity, specific heat capacity and thermal conductivity of low concentration hybrid nanofluids. The present work also focused on comparing these properties and the thermal enhancements with other low concentration mono nanofluids. The nanofluids used for present work are Al2O3-CuO, Al2O3 and CuO combined with a 60:40 mixing ratio of distilled water and ethylene glycol acting as the base fluid. The volume concentration at which the nanofluids are prepared are fixed 0.02%, 0.04% and 0.06%. The findings show that the thermal conductivity of the hybrid samples is higher compare to its mono counterparts. The thermal conductivity of Al2O3-CuO showed enhancements by 2.3% when compared to CuO and by 3.6% when compared to Al2O3. By using the prepared samples as a working fluid in a compact fin and tube heat exchanger test rig, an experimental investigation is conducted. This is to observe any form of improvements in terms of heat transfer by comparing the experimental results of the hybrid nanofluids with its respective mono counterparts. Based on the experiments conducted, it is found that the use of Al2O3-CuO hybrid nanofluids as a working fluid in a compact heat exchanger test rig showed an increase in both Nusselt number and average heat transfer coefficients by 6.7% and 7.2% respectively when compared to CuO nanofluids and by 17.9% and 12.1% respectively when compared to Al2O3 nanofluids.