One Question per Day (February 2)

⑴　The driving force of fluid flow. There are two types of forces driving fluid flow: one is free flow ； One is forced flow. Forced flow heat transfer is usually more intense than free flow heat transfer. 　　⑵　Whether there is a phase change in the fluid. Generally, convective heat transfer for the same fluid is stronger when there is a phase change than when there is no phase change. 　　⑶　Flow regime of the fluid. Due to the intense mixing of fluid elements under turbulence, heat exchange is more vigorous in turbulent flow than in laminar flow. 　　⑷　Impact of geometric factors. The shape and size of the solid surface in contact with the fluid, as well as the relative position between the fluid and the solid, all affect convective heat transfer. 　　⑸　Physical properties of fluids. Different fluids have varying densities, viscosities, thermal conductivity, specific heats, and latent heats of vaporization, all of which affect the heat exchange between the fluid and the solid wall. Note: Substances exist in three phases: solid, liquid, and gas. Phase change refers to the change in their state.

1. Causes of convective motion and flow state； 　　2. Physical properties of fluids (varying with type, temperature, and pressure) ； 　　3. Shape, size, and relative position of the heat transfer surfaces ； 　　4. Whether there is a phase change in the fluid (such as transformation between gas and liquid states).

(1) Avoid using main steam pipes and reheat steam pipes with large wall thicknesses and diameters to save on investment. (2) Reduce pipeline pressure loss.

Answer: Convective heat transfer is a relatively complex process of energy and mass exchange; therefore, there are many factors that influence the extent of convective heat transfer. The main ones include: (1) Fluid flow regime – Any flow of fluid driven by external forces is considered forced motion； Natural convection caused by different temperatures in various parts of the fluid is called free motion. The convective heat transfer coefficient of a fluid in forced motion is greater than that in free motion ； The heat transfer coefficient of a fluid in a turbulent state is greater than that of a fluid in a laminar state. This is because when the fluid flow velocity is high, the disturbances are intense, thereby enhancing heat transfer. (2) Whether phase change occurs in the fluid: The convective heat transfer coefficient is much higher when a liquid vaporizes or a gas condenses, compared to when no phase change takes place. As in the case of boiling water, as bubbles are continuously formed, grow, and break away from the wall surface, the cold fluid keeps rushing toward the wall, causing intense turbulence in the fluid and significantly increasing the heat transfer rate. (3. Physical properties of fluids: Different types of fluids have different physical properties, which also result in significant variations in the convective heat transfer coefficients. Generally, fluids with a high thermal conductivity also have a high convective heat transfer coefficient ； Fluids with high specific heat capacity and density have larger convective heat transfer coefficients ； When the viscosity of a fluid is high, its convective heat transfer coefficient is low. (4. Flow direction of the fluid relative to the heated tube: The convective heat transfer coefficient is higher when the fluid flows transversely across the tube compared to when it flows longitudinally across the tube.) (5) Structural characteristics of the heat-exchanging surface: Pipe diameter, pitch, and arrangement pattern all have an impact on the heat transfer coefficient. Small diameter, small pitch, high convective heat transfer coefficient ； Tubes arranged in a staggered pattern have a higher convective heat transfer coefficient than those arranged in a straight line. Because these measures all cause strong fluid disturbances, thereby enhancing heat transfer. 1# Stand in the east

　There are mainly five factors that affect convective heat transfer: ⑴ The driving force of fluid flow. There are two types of forces driving fluid flow: one is free flow ； One is forced flow. Forced flow heat transfer is usually more intense than free flow heat transfer. 　　⑵　Whether there is a phase change in the fluid. Generally, convective heat transfer for the same fluid is stronger when there is a phase change than when there is no phase change. 　　⑶　Flow regime of the fluid. Due to the intense mixing of fluid elements under turbulence, heat exchange is more vigorous in turbulent flow than in laminar flow. 　　⑷　Impact of geometric factors. The shape and size of the solid surface in contact with the fluid, as well as the relative position between the fluid and the solid, all affect convective heat transfer. 　　⑸　Physical properties of fluids. Different fluids have varying densities, viscosities, thermal conductivity, specific heats, and latent heats of vaporization, all of which affect the heat exchange between the fluid and the solid wall. Note: Substances exist in three phases: solid, liquid, and gas. Phase change refers to the change in their state.

1. Fluid properties. 2. Fluid flow rate. 3. Roughness of the inner wall and inner diameter. 4. Heat exchange temperature difference: Station 1# is located in the east

(1) Avoid using main steam pipes and reheat steam pipes with large wall thicknesses and diameters to save on investment. (2) Reduce pipeline pressure loss.

Since convective heat transfer involves heat transfer through both conduction and thermal convection, any factors that affect these two modes of heat transfer will also influence the convective heat transfer process. The factors affecting convective heat transfer are as follows: (1) The cause of fluid flow. Since the causes of the two types of flow differ, the velocity fields within the fluid also vary, resulting in different heat transfer patterns. (2) Whether there is a phase change in the fluid: In the absence of a phase change, heat exchange occurs due to changes in the specific heat of the fluid ； When phase change occurs, the latent heat of the fluid usually plays a major role, hence the heat transfer laws are different from those in the absence of phase change. (3) Fluid flow regime: Viscous fluids exhibit two different flow regimes—laminar flow and turbulent flow. Under constant other conditions, the heat transfer intensity in turbulent flow is greater than that in laminar flow. (4) Geometric factors of the heat exchange surface: Factors such as the shape, size, condition, and geometric arrangement of the heat exchange surface all influence the heat exchange behavior. (5) Physical properties of the fluid: The density, dynamic viscosity, thermal conductivity, and specific heat at constant pressure of a fluid all affect the velocity distribution within the fluid as well as heat transfer, thereby influencing convective heat exchange.

Convection heat transfer is a phenomenon of heat transfer that occurs during fluid flow; it relies on the movement of fluid particles to transfer heat. The rate equation for convection heat transfer is Q = aAΔt, where a is the convective heat transfer coefficient, A is the heat transfer area, and Δt is the temperature difference. Determining the convective heat transfer coefficient a is key to solving problems related to convection heat transfer. The factors that influence the convective heat transfer coefficient a include: 1. Physical properties of the fluid, such as density, viscosity, thermal conductivity, and specific heat capacity; 2. The causes of fluid convection, such as natural convection and forced convection; 3. The flow state of the fluid, namely turbulent or laminar flow; 4. Phase changes in the fluid, whether there is any phase change or not; 5. The shape, size, and relative position of the heat transfer surface

In general, the factors that affect convective heat transfer are mainly the following four aspects: (1) Physical properties of the fluid. The physical properties of a fluid, that is, the type of fluid, have a significant impact on convective heat transfer. For example, a hot object cools down faster in water than in air at the same temperature. The physical parameters that affect convective heat transfer are commonly referred to as the thermophysical properties of the fluid. It mainly includes: density ρ, thermal conductivity λ, specific heat cv, dynamic viscosity u, etc. For each fluid, these parameters have specific values; they are typically functions of temperature, and for gases, they are also related to pressure. 　　 (II) Causes of fluid motion 　　 Based on the reasons for fluid motion, it can be divided into two categories. One type is natural motion caused by the different densities in the hot and cold parts of the fluid ； Another type is forced motion caused by external forces, such as those from wind turbines or water pumps. Under normal circumstances, the heat transfer intensity of forced convection is much higher than that of natural convection. 　　 When a fluid is subjected to forced motion, natural motion also occurs. When the speed of forced convection is very high, the effect of natural convection on heat transfer can be ignored ； When the forced convection is not very strong, the influence of natural convection increases relatively and must be taken into account; this condition is known as mixed convective heat transfer. 　　 (III) State of fluid motion As known from the course \"Fluid Mechanics,\" the state of fluid motion can be divided into laminar flow and turbulent flow. Laminar flow occurs when the Reynolds number is Re≤2300, and all parts of the fluid move parallel to the walls of the flow channel without interfering with each other ； Turbulence occurs when Re≥104, with the flow in a chaotic and irregular state; only a very thin boundary layer flow exists near the wall surfaces of the flow channel. When the fluid is at 2300