Heat transfer augmentation techniques (passive, active or a combination of passive and active methods) are commonly used in areas such as process industries, heating and cooling in evaporators, thermal power plants, air-conditioning equipment, refrigerators, radiators for space vehicles, automobiles, etc. Passive techniques, where inserts are used in the flow passage to augment the heat transfer rate, are advantageous compared with active techniques, because the insert manufacturing process is simple and these techniques can be easily employed in an existing heat exchanger. In design of compact heat exchangers, passive techniques of heat transfer augmentation can play an important role if a proper passive insert configuration can be selected according to the heat exchanger working condition (both flow and heat transfer conditions). In the past decade, several studies on the passive techniques of heat transfer augmentation have been reported. The present paper is a review on progress with the passive augmentation techniques in the recent past and will be useful to designers implementing passive augmentation techniques in heat exchange. Twisted tapes, wire coils, ribs, fins, dimples, etc., are the most commonly used passive heat transfer augmentation tools. In the present paper, emphasis is given to works dealing with twisted tapes and wire coils because, according to recent studies, these are known to be economic heat transfer augmentation tools. The former insert is found to be suitable in a laminar flow regime and the latter is suitable for turbulent flow. The thermohydraulic behaviour of an insert mainly depends on the flow conditions (laminar or turbulent) apart from the insert configurations. The present review is organized in five different sections: twisted tape in laminar flow; twisted tape in turbulent flow; wire coil in laminar flow; wire coil in turbulent flow; other inserts such as ribs, fins, dimples, etc.

In 21st century, the energy demand has grown incredibly due to globalization, human population explosion and growing megacities. This energy demand is being mostly fulfilled by fossil-based sources, which are non-renewable and a major cause of global warming. Energy from these fossil-based sources is cheaper, however challenges exist in terms of climate change. This makes renewable energy sources more promising and viable for the future. Hydrogen is a promising renewable energy carrier for fulfilling the increasing energy demand due to its high energy density, non-toxic and environment friendly characteristics. It is a non-toxic energy carrier as combustion of hydrogen produces water as the byproduct whereas other conventional fuels produce harmful gases and carcinogens. Because of its lighter weight, hydrogen leaks are also easily dispersed in the atmosphere. Hydrogen is one of the most abundant elements on Earth, yet it is not readily available in nature like other fossil fuels. Hence, it is a secondary energy source and hydrogen needs to be produced from water or biomass-based feedstock for it to be considered renewable and sustainable. This paper reviews the renewable hydrogen generation pathways such as water splitting, thermochemical conversion of biomass and biological conversion technologies. Purification and storage technologies of hydrogen is also discussed. The paper also discusses the hydrogen economy and future prospects from an Indian context. Hydrogen purification is necessary because of high purity requirements in particular applications like space, fuel cells etc. Various applications of hydrogen are also addressed and a cost comparison of various hydrogen generation technologies is also analyzed. In conclusion, this study can assist researchers in getting a better grasp of various renewable hydrogen generation pathways, it's purification and storage technologies along with applications of hydrogen in understanding the hydrogen economy and its future prospect.

A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impinging flows.