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I. Heat transfer coefficient α: According to Newton’s law of cooling, dQ = α A△t. The unit of the heat transfer coefficient α is W/(m2 *℃). Its physical meaning is the amount of heat transferred by heat conduction per unit time, per unit area of heat transfer, when the temperature difference is 1 K. Experiments have shown that the factors affecting the heat transfer coefficient include: ① the flow velocity of the fluid: Heat conduction within the heat transfer boundary layer is the main factor determining convective heat transfer. Clearly, increasing the flow velocity can thin the heat transfer boundary layer, thereby increasing α and enhancing the convective heat transfer process. ②Convection condition of the fluid: whether to use natural convection or forced convection. Obviously, the flow velocity of the fluid under forced convection is higher than that under natural convection. ③Types of fluids ； Liquids, gases, vapors. ④Properties of fluids: Those that have a significant impact include the specific heat, thermal conductivity, density, viscosity, etc., of the fluid. For fluids with a high thermal conductivity, the thermal resistance of the heat transfer boundary layer is low, resulting in a higher heat transfer coefficient. For fluids with high viscosity, at the same flow velocity, the Re number is low; consequently, the heat transfer boundary layer is thicker, and the heat transfer coefficient is smaller. ⑤Shape, position, and size of the heat transfer surface: Different shapes of heat transfer surfaces, such as circular tubes, flat plates, or tube bundles ； Is it inside the tube or outside the tube? ； Should it be placed vertically or horizontally? ； And different pipe diameters and lengths also have an impact on α. Summarizing the above influencing factors, the heat transfer coefficient α can be expressed in the following indefinite equation: α = f(u, d, ρ, μ, t, cp, …). The conditions of various heat transfer processes vary greatly, and the factors that affect α are not identical across them. Common heat transfer processes can be divided into two main categories: 1. Heat transfer processes without phase changes in the fluid: (1) Heat transfer by forced convection of the fluid; (2) Heat transfer by natural convection of the fluid. 2. Heat transfer processes with phase changes in the fluid: (1) Heat transfer due to vapor condensation; (2) Heat transfer due to liquid boiling. Empirical formulas for the convective heat transfer coefficient in the absence of phase changes: For forced turbulent flow in a circular straight tube, the heat transfer coefficient is given by α = 0.023 * (λ/d) * (ρdu/μ)^0.8 * (cμ/λ)^b. When the fluid is heated, b = 0.4; when the fluid is cooled, b = 0.3. (Re > 10000, 0.7)