Methods to improve the heat transfer coefficient of heat exchangers

Thread Content

Changing the flow pattern of a fluid: (1) Increasing the flow velocity – Raising the flow velocity can alter the flow state and increase the degree of turbulent fluctuations. In shell-and-tube heat exchangers, dividing the tube side and shell side flow paths is one of the measures taken to increase flow velocity, lengthen the flow path, and introduce turbulence. In turbulent flow within the pipe, increasing the flow velocity has a significant effect on enhancing heat transfer, but it should be noted that increasing the flow velocity is also limited by various factors. Therefore, various factors should be weighed in design or actual use to select the optimal flow rate or the flow rate permitted by the fluid transfer machinery. (2) Jet impact: This is a method in which a fluid is directly sprayed onto a solid surface through a circular or slit-shaped nozzle for cooling or heating. Since the fluid impacts the solid wall directly, the flow path is short and the boundary layer is thin, resulting in a significantly increased convective heat transfer coefficient. When a liquid jet impacts the heating surface, if the heat flux density is already high enough to cause boiling, it becomes two-phase jet impingement heat transfer. Experiments show that at this point, not only can the boiling heat transfer coefficient be increased, but the burnout point can also be delayed, thereby significantly raising the critical heat flux value. (3) Insertion of inserts: Placing inserts of various types such as wires, metal helical coils, disc-shaped components, twisted iron, or airfoils inside the tube or surrounding it externally can enhance turbulence and disrupt the flow boundary layer, thereby increasing heat transfer. By inserting twisty iron perturbators made from thin metal strips into the tube, a strong rotational flow is generated in the fluid, thereby enhancing heat transfer. If it can make close contact with the tube wall during insertion, it can still serve as fins to increase the heat transfer surface area. Numerous experimental studies have shown that the addition of inserts significantly enhances forced convection heat transfer, but it also introduces operational issues such as increased flow resistance, as well as easy clogging and scaling of the channels. When using inserts, they should be placed along the entire length of the pipeline to maintain enhanced heat transfer throughout the whole process. Furthermore, when selecting the form of the insert, consideration should be given to enhancing heat transfer under low resistance. (4) Addition of a rotating flow device: The centrifugal force generated by the rotating flow causes secondary circulation in the fluid, thereby enhancing heat transfer. Some of the aforementioned inserts, such as twisted iron and metal spiral wires, can also generate rotational flow in addition to their inherent properties. Mention should be made here of some components or devices specifically designed to generate rotational flow. For example, an eddy current generator, which enables a fluid to enter a tube at a certain pressure in a tangential direction and undergo intense rotational motion. Studies show that the degree of heat transfer enhancement by vortices is related to the Reynolds number. At a certain heat source temperature, the convective heat transfer coefficient increases with the Re value, reaches a certain maximum value, and then decreases. In applications, the actual Re value should be controlled to be close to the critical Re value at which the convective heat transfer coefficient is maximized, in order to make full use of the effects of rotational flow. In addition to fluid rotation, there is also the case of rotating heat transfer surfaces; when the pipe rotates around different axes, the secondary circulation generated by centrifugal force, shear stress, gravity, and buoyancy can enhance heat transfer. Pipe rotation significantly enhances laminar heat transfer, while the effect is minimal in turbulent flow. Experiments on subcooled boiling and large-space boiling show that for pipes equipped with spiral inclined surfaces and tangential groove vortex generators, the boiling heat transfer coefficient or critical heat load can be increased. (5) Relying on external energy: There are generally three approaches: ① Using mechanical or electrical methods to cause vibration in the heat transfer surface or in the fluid, or to mix the fluid thoroughly through stirring. Tests show that vibration has a certain effect on both free-flow heat transfer and forced-flow heat transfer. The effect on boiling heat transfer is not significant, but during fluid vibration, it can markedly increase the critical heat load for intense boiling in large volumes. This method presents certain difficulties in practical application for large heat exchange equipment. A mechanical drive is used to activate the stirrer, thereby enhancing convective heat transfer through thorough mixing of the fluid; this approach yields significant results, which is why it is widely used, especially for fluids with high viscosity. ②Acoustic or ultrasonic waves are applied to the fluid, causing it to alternate between compression and expansion in order to increase pulsation and enhance heat transfer. The results of experimental studies by various researchers show that for liquids or gases, acoustic effects are significant only when flow is in laminar or transitional regime within the tube. It has a minimal impact on heat transfer in large-scale bubble boiling, but it improves heat transfer significantly in transition boiling or film boiling. It is effective for both condensation heat transfer and free-flow heat transfer. In the practical application of acoustic enhancement measures, it is necessary to address how to transmit acoustic or ultrasonic vibrations more effectively into the heat exchange equipment. ③Effect of electromagnetic fields. A high voltage is applied to the fluid involved in heat exchange to create a non-uniform radial electric field; such an electrostatic field can induce mixing of the dielectric fluid near the heat transfer surface, thereby enhancing convective heat transfer. Tests show that the enhancement effect is significant for free-flow heat transfer, film boiling heat transfer, and condensation heat transfer. If magnet powder is added to the fluid, the magnetic field can enhance heat transfer even at high Re numbers. For example, by adding magnet powder to water or oil, the heat transfer coefficient can be increased by more than 50% under the influence of a magnetic field. Modifying the physical properties of the convective fluid: The physical properties of a fluid have a significant impact on its convective heat transfer coefficient. Generally, fluids with higher thermal conductivity and specific heat capacity also exhibit higher heat transfer coefficients. For example, in cooling equipment, using water cooling can significantly reduce the volume compared to air cooling, because the α value between air and a wall surface ranges from 1 to 60 W/(m²·°C), while the α value between water and a wall surface ranges from 200 to 12,000 W/(m²·°C). Another way to alter certain properties of a fluid is by adding some additives to it; this has become a new area of research on additive-enhanced heat transfer that has emerged over the past two to three decades. The additive can be solid or liquid; it combines with the heat transfer fluid to form gas-solid, liquid-solid, vapor-liquid, and liquid-liquid mixed flow systems. Changing the heat exchange surface conditions: The properties, shape, and size of the heat exchange surface have a significant impact on the convective heat transfer coefficient. Heat transfer can generally be enhanced through the following methods: (1) Increasing wall roughness. Increasing wall roughness is beneficial not only for forced convection heat transfer inside the tube, but also for boiling and condensation heat transfer as well as forced convection heat transfer outside the tube. The effect of the same roughness on heat transfer performance varies under different flow and heat exchange conditions. Increasing roughness also leads to an increase in flow resistance, which should be taken into account in industrial applications. (2) Changing the shape and size of the heat exchange surface: To increase the convective heat transfer coefficient, various specially shaped tubes and surfaces with grooves can also be used, such as elliptical tubes, spiral tubes, corrugated tubes, tubes with variable cross-sections, and tubes with longitudinal grooves. For the same cross-sectional area, the equivalent diameter of elliptical tubes is smaller than that of circular tubes; therefore, their heat transfer coefficient is higher. Apart from a slight increase in the heat transfer area, in other shaped tubes, due to the changes in surface geometry, the fluid flow continuously changes its direction and velocity, which intensifies turbulence and reduces the thickness of the boundary layer; thus, heat transfer is enhanced. For low-finned threaded tubes, they also serve to thin the condensation film during condensation heat transfer; thus, they are highly beneficial for the condensation of organic working fluids (such as Freon). The micro-ribbed tubes developed based on low-ribbed tubes are even more conducive to the condensation heat transfer of low-boiling-point organic media such as Freon; examples include Japan’s C-tubes and China’s DAC tubes. In the case of vertical condensation, when vertical grooved tubes are used, the surface tension of the liquid pulls the condensate at the wave crests into the wave troughs, forming an extremely thin condensate film at the crests. Meanwhile, the condensate is drained from the troughs; this thus enhances the condensation heat transfer. (3) Improving the surface structure: Sintering, electrical discharge machining, or cutting of metal tubes can result in the formation of a very thin porous metal layer on their surface, thereby creating porous tubes that enhance boiling and condensation heat transfer. For example: the high heat flux tubes used for boiling heat transfer in the United States, the E-type tubes in Japan, the T-type tubes in Germany, and the DAE tubes in China, etc. Furthermore, in boiling heat-transfer fluids, a porous object can be placed on the heating surface; by continuously removing steam through this porous heating surface, via a process known as \"suction,\" film boiling heat transfer is improved. (4) Surface coating: During condensation heat transfer, a material with low surface tension, such as polytetrafluoroethylene, can be applied to the heat exchange surface to induce bead-shaped condensation, which helps to increase the heat transfer coefficient. For boiling heat transfer, based on the physical properties of the liquid being heated, a thin film of a certain substance with an appropriate thickness can be applied to the heating surface to make it a non-wetting surface, thereby significantly increasing the boiling heat transfer coefficient. In solar energy utilization, a thin layer of selective material is applied to the heat-absorbing surface of the collector to increase its absorption of sunlight and reduce its emissivity, thereby enhancing the absorption of radiant heat and minimizing heat loss.