Hydrogen is a promising energy career in the future carbon-neutral society. In December 2014, Toyota motors has started to sell a commercial fuel cell vehicle (FCV) into Japanese market and has been increasing its production year by year. In order to disseminate the FCVs, the hydrogen refueling station (HRS) should be placed everywhere in the society.
The current FCV has 5kg of hydrogen at 70MPa in tank pressure. Therefore, the HRS has to handle hydrogen with higher pressure up to 100MPa to refill it to FCVs. Currently, the charging time to fill up the H2 tank is expected to be 3 min or shorter. However, such a rapid refueling process causes temperature rise in the H2 tank by adiabatic compression. The limit temperature of the H2 tank is 85oC because of the melting point of a bonding agent for carbon fibers of the tank. To avoid undesirable temperature rise, the recent HRSs are equipped with a precooling device that cools the hydrogen down to -40oC in advance. However, this precooling process causes the other problems. One is the overcharge of hydrogen.
Since the hydrogen gas at low temperature is injected into tank up to target pressure at the end of refueling process, the tank pressure increases gradually due to heat transfer from the surrounding. This pressure rise by heat transfer sometimes exceeds the limit pressure of the tank. The second problem is a frost formation around the injection nozzle and the receptacle. Sometimes, ice is formed and the hydrogen nozzle is stuck to the receptacle. For safety operations of HRS, these thermal problems must be solved.
Our research effort is devoted to the first problem. To ensure the safety of FCVs during refueling hydrogen, transient pressure and temperature in tank should be predicted with sufficient accuracy. The first step is to collect accurate thermophysical property data for hydrogen in a wide range of pressures and temperatures. We have been measuring thermodynamic and transport properties of hydrogen up to 100MPa and 500oC to develop a reliable database. Based on this hydrogen thermopysical proerty database, we have been developing a useful software for dynamic simulation of HRS which predicts flow rate, temperatures and pressures of HRS and H2 tank of FCV. Some typical thermo-technical problems with hydrogen refueling process are introduced in the presentation.
Pool boiling heat transfer centers on the nucleation, growth and departure of a bubble on a surface through latent heat. During this process, heat is transferred through (i) microconvection, (ii) microlayer evaporation, (iii) transient conduction, and (iv) contact line heat transfer mechanisms. The heat transfer in each of these processes relies on the region surrounding the bubble, often referred to as the influence region. Enhancement strategies are devised to improve the local nucleation and rewetting mechanisms in this region which have resulted in significant improvements in either critical heat flux (CHF) or heat transfer coefficients (HTC). The enhancement techniques (tall porous structures, wicking microstructures, micro-grooves/ridges, etc.) utilize the aforementioned heat transfer mechanism to increase the performance levels. New heat transfer mechanisms have been identified recently that enable us to make pool boiling surfaces that dissipate exceptionally high heat fluxes, in excess of 400 W/cm2 with heat transfer coefficient in excess of 1 MW/m2 0C. The talk will present a progression of our understanding in this exciting area and provide directions for future research needs.
Due to world population growth, growing industrial development, improvements in living standards, emerging technologies (such as electric vehicles) and the growth in domestic and industrial use of modern equipment, the current growing trend in energy demand and consumption will continue long into the future. In this regard, combustion devices such as gas turbines for power, aero-engines and i.c. engines for transport will still be used. Design, improvements and operation of such combustion devices will require a good understanding of fluid flow, combustion and heat transfer. Increase use of new bio-fuels and other derived fuels in combustion systems also requires new knowledge. For example,Hydrogen in this context is considered to be the clean fuel of the future which does not contribute to greenhouse gases. Injection of Hydrogen, generated from excess electricity, into national transmission networks of various countries is being considered as a way of de-carbonisation. Such developments will have implications in the operation, and safety of gas turbine plants and domestic boilers and many other equipment.New technologies require analytical and evaluation techniques; in this regard Computational Fluid Dynamics (CFD) is now being used in many applications and its use for the modelling of combustion devices has become very popular. CFD has the ability to predict combustion emissions, heat transfer and performance of many combustion devices and it has become a useful research and development tool. Recent advances in Large Eddy Simulation (LES) technique in CFD and the availability of high performance computing (hpc) resources has made CFD calculations to be more accurate, simulations very realistic and useful for the improvement of many combustion processes. CFD modelling of combustion can also be applied to model vastly complex phenomena such as explosions and safety related process. A variety of CFD combustion modelling applications in flames, I.C. engines, explosion situations etc. are presented and described in this keynote lecture to illustrate the current status of advanced CFD applications in combustion.