@article{SalamaKouDawoudetal., author = {Salama, Amgad and Kou, Jisheng and Dawoud, Belal and Simonson, Carey}, title = {A modeling approach for capillary-driven flow of a wetting fluid in a rectangular open microchannel of arbitrary axisymmetric width profile}, series = {International Journal of Thermal Sciences}, volume = {195}, journal = {International Journal of Thermal Sciences}, publisher = {Elsevier}, issn = {1290-0729}, doi = {10.1016/j.ijthermalsci.2023.108622}, pages = {17}, abstract = {Microchips are intensively used in almost all nowadays electronic devices. With the continuous advancement of our technologies, they get smaller in size than ever before. They generate high-intensity heat loads that need to be transported effectively such that they may function properly. Heat pipes have proven to be very effective in transporting relatively large heat loads from miniature components. They are of seamless structure that involves a working fluid capable of evaporation and condensation at the working temperature of the electronic chips. The working fluid is derived to move from the condenser to the evaporator via multiple microgrooves using capillary forces. It is important that the condensate reaches the evaporator at a proper rate such that no dry-out or flooding occur. In this work, we are particularly interested in the case of capillary-driven flows in rectangular microchannels. A generalized model is developed that works for axe-symmetric rectangular channels of arbitrary, moderately varying width profiles. It also accounts for any contrast of viscosity between the liquid and the vapor under isothermal conditions. The model shows to reduce to the special case of imbibition in straight and uniform microchannels, for which comparisons with experimental and modeling works show an excellent match. Cases representing linearly and quadratically varying converging/diverging width profiles have been explored. It is found that the viscosity ratio has a significant influence on the rate at which the meniscus advances. The model also negates the common practice found in the literature of using the formula developed for imbibition rates in capillary tubes for rectangular microchannels by replacing the diameter of the tube with the hydraulic diameter. It is also found that the channel profile has an influential effect on the imbibition rates. For tapered microchannels, the capillary force increases along the channel length while it decreases for diverging ones. It is interestingly demonstrated that, for quadratically tapered microchannel, the speed of the meniscus increases towards the end of the microchannel compared with linearly varying microchannels. On the other hand, for diverging microchannels, the speed of the meniscus decreases due to the increase in the cross-sectional area. Computational fluid dynamics (CFD) analysis has been conducted to provide a framework for confirmation and verification for which very good match has been established, which builds confidence in the modeling approach.}, language = {en} } @article{WeindlerDawoud, author = {Weindler, Jeremy and Dawoud, Belal}, title = {Experimental investigation of a small-scale continuous flow ohmic heating reactor for a highly effective conversion of renewable electricity surpluses into heat}, series = {Applied Thermal Engineering}, volume = {242}, journal = {Applied Thermal Engineering}, publisher = {Elsevier}, issn = {1359-4311}, doi = {10.1016/j.applthermaleng.2024.122401}, pages = {15}, abstract = {Sector coupling is expected to play a decisive role as a key technology to realize the energy transition towards green energy supply. This work introduces, therefore, a comprehensive experimental study on a new power-to-heat system based on the continuous-flow, ohmic heating principle, which aims at utilizing renewable electricity in green processes. The ohmic heating technology, which is widely applied in the food industry, has been adapted and evaluated for delivering heat and domestic hot water to a single-family house. The introduced prototype is designed to convert renewable electricity into heat in the power range between 1000 and 3000 W with a typical electrical household installation (230 V, 50 Hz and 16 A fuse). A dedicated test rig has been developed and a comprehensive experimental campaign has been carried out to investigate the performance of the introduced system under typical operating conditions of a single-family house. Both the conversion efficiency and the dynamics of the thermal response of the introduced system have been assessed. It turned out that, the temperature lift between the return and supply of the reactor, and consequently, the conversion efficiency of electrical into thermal energy, are independent on the return temperature, for a fixed value of the volume flow rate of the electrolyte solution. A remarkable steady-state conversion efficiency of 97.5\% has been measured. In addition, the dynamic response of the ohmic heating system has been found to be strongly dependent on the desired temperature lift. By an implemented controller, 95\% of the steady state temperature lift of 44.5 K can be realized in 50 s, which marks an exceptionally rapid thermal response. The obtained results pave the way towards a field test study of the developed technology in a real household installation as well as to extend the focus on new fields such as green chemical processes.}, language = {en} }