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Energy and the First Law of Thermodynamics
Published in Kavati Venkateswarlu, Engineering Thermodynamics, 2020
Basically, heat is transferred by three modes: conduction, convection, and radiation. Conduction is the type of heat transfer from the particles of higher energy levels of a substance to the adjacent particles of lower energy levels resulting from the interaction between particles. Convection is the type of heat transfer between a solid surface and an adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion. Examples include air circulation with a fan or blower in a room or flow through heat exchangers such as a condenser in which water/air is circulated through piping to cool the working fluid. Radiation, on the other hand, is the type of heat transfer that takes place due to the emission of electromagnetic waves (or photons). Thermal radiation is the energy emitted by matter that is at a non-zero temperature. Radiation, unlike the other two modes, can even happen in vacuum (most efficiently) and does not require any medium; however, the emission of the radiation and the absorption require a substance to be present. Although the radiation from solid surfaces is of importance all the time, liquids and gases also emit radiation.
System Selection and Optimization
Published in Samuel C. Sugarman, HVAC Fundamentals, 2020
Thermodynamics is the science of heat energy and the study of how heat energy can be changed from one form of energy to another. One of the laws of thermodynamics states that heat energy flows from a higher level to a lower level. When this law is applied to heat recovery systems it tells us that the waste heat in a fluid such as air, flue gas, steam, refrigerant, brine, or water from a heat-generating process can be captured and transferred to a cooler fluid for use in another process. The intent of heat recovery is to reduce energy costs by supplementing the energy required to fuel the process or comfort system. Conduction, convection, and radiation are the three means of heat transfer. Conduction is the transfer of heat from on substance to another when each substance is in direct physical contact with the other. An example of conduction is a human hand on a cold pipe. Warmth from the skin is transferred to the pipe. Convection is heat transfer by movement of a fluid over an object. Convection is demonstrated when heated air flows into a room and warms the occupants. Radiation is heat transfer by waves transmitted from the source of the heat to an object receiving the heat waves without heating the space. Examples are when the sun’s rays heat a glass window or when a person is warmed by the heat waves from a fire or infrared heater.
Measurement Techniques for Thermal Conductivity of Nanofluids
Published in Chetan Keswani, Intellectual Property Issues in Nanotechnology, 2020
For fluid heating and cooling, one of the extremely crucial aspects of various applications is heat transfer, for instance in microelectronics, transportation, power generation, manufacturing, chemical processes, etc. It is well known that a tiny enhancement in the practical applications will increase equipment life, save energy, and reduce processing time.
Optimisation techniques for solar drying systems: a review on modelling, simulation, and financial assessment approaches
Published in International Journal of Sustainable Energy, 2023
Baibhaw Kumar, Gábor Szepesi, Zoltán Szamosi
Optimised by efficiency and working conditions, the dryers could help the farmers significantly enhance their revenue. There are two broad classifications of solar dryers based on airflow: the active type (forced convection) and the passive type (natural convection). The user needs to understand the drying requirements of the product before selecting and fabricating the solar dryer (Sharma, Chen, and Vu Lan 2009). Conduction, convection, and radiation are the three basic modes of heat transfer. Solar dryers primarily work on the combinations of these modes of heat transfer. Several factors are vital in designing or fabricating solar dryers: portable design, thermally efficiency, and cost-effectiveness. A study by Chavan et al. (2021a) reveals that a conduction mechanism could consolidate the innovations in solar drying techniques in the case of natural convective and direct-type dryers. Airflow and temperature are the most important parameters affecting the drying phenomenon, among several other factors. The other parameters, which govern the optimisation, could be the sun’s radiation, the air’s humidity, the load of product, and resistance to airflow. Spoilage of crops and food products is a big concern for rural farmers with limited sources. Thus, a user-friendly low-cost solar drying solution could be a boon for them. Air and temperature control circulation should be of prime concern in manufacturing such dryers. Hence, the dryers, which are scientifically optimised, are better performing (Prakash and Kumar 2013).
Shape-stabilized composite phase change material for thermal insulation of cotton fabrics with sandwich structure
Published in The Journal of The Textile Institute, 2023
Wei Zhang, Yibo Zhang, Jiming Yao, Sainan Wei, Kailiang Lu
PCMs can change their state within a small temperature range and absorb or release large amounts of latent heat. The excellent thermal energy storage capacity has led to applications in solar power generation, building energy management, waste heat recovery systems, thermal management of electronic devices, and preparation of functional fabrics (Aditya et al., 2021; Liu et al., 2020). Clothing should protect the human body from the harmful effects of extreme temperature changes, so the development of heat-insulating fabrics has become the focus of industry research. Conduction, radiation, and convection are the three main modes of heat transfer. Correspondingly, researchers have developed barriers, reflection, and radiation insulation materials (Zhang et al., 2022). Among them, barrier insulation materials achieve the purpose of heat insulation through significant resistance to heat transfer (Wang et al., 2008; Zhang et al., 2022). Compared to other energy storage materials, phase change materials have the advantages of easy availability of raw materials, high storage density, durability, and a variety of shapes, making them the preferred choice for heat storage and insulation materials (Kurnia & Sasmito, 2018; Xu et al., 2017).
Thermal aspects of radiation in Casson fluid with nonlinear stretching surface: non-similar solutions
Published in Waves in Random and Complex Media, 2022
Muavia Mansoor, M. Shoaib Kamran, Qazi Mahmood Ul-Hassan, Muhammad Irfan
Thermal conductivity is a physical property that depends upon temperature. Thermal conductivity is defined as the rate of heat transfer through a unit length of material per unit temperature difference. According to Batchelor and Animasaun, thermal conductivity varies with variation in temperature [18,19] Large amount of work has been done that explains the effect of thermal conductivity on flowing fluids. Chaim et al. observed thermal conductivity in boundary layer flow of 2D Newtonian fluid in a porous medium [20]. Attia [21] presented unsteady flow between two parallel plates with variable thermal conductivity and viscosity. Jawad et al. used the Attia model for observing both thermal conductivity and viscosity of flow in vertical channels. He observed that heat transfer and fluid flow decrease with an increase in thermal conductivity [22]. Gbadeyan et al. [23] investigated the effects of thermal conductivity on viscous, incompressible Casson nanofluid. After solving the system of equations, the effects of different physical parameters like Sherwood number and Nusselt number were observed and plotted graphically. T. Hayat et al. [24] investigated mixed convection flow over a stretching surface with variable thermal conductivity and observed the effects of governing parameters. T. Hayat also discussed Casson fluid over a stretching sheet with the effect of thermal conductivity and presented a series of solution [25]. Mondal and Pal [26] investigated the effects of thermal conductivity and temperature-dependent viscosity in MHD strained fluid flow. More detail about current studies can be reported in references [27–31].