How to improve the heat transfer coefficient of heat exchanger?
Changing fluid flow
Increase the flow rate
Increasing the velocity can change the flow state and increase the turbulence fluctuation. For example, tube side and shell side split in shell and tube heat exchanger is one of the measures to increase flow rate, process length and disturbance. However, it should be noted that the increase of flow rate is limited by various factors. Therefore, various factors should be considered in the design or practical use to select the best flow rate or the flow rate allowed by the fluid conveying machinery.
This is a method of cooling or heating the solid surface directly by spraying the fluid through a circular or slit shaped nozzle. Because the fluid directly impinges on the solid wall, the flow path is short and the boundary layer is thin, so the convective heat transfer coefficient increases significantly. When a liquid jet impinges on a heated surface, if the heat flux is high enough to produce boiling, it is called impingement heat transfer of two-phase jet. The experimental results show that not only the boiling heat transfer coefficient can be increased, but also the burnout point can be delayed, and the critical heat flow value can be significantly increased.
Many kinds of inserts, such as metal wire, metal spiral ring, disk-shaped component, twist iron and wing shaped object, are placed inside or outside the tube, which can enhance the disturbance and damage the flow boundary layer and increase the heat transfer. For example, the twisted twisted twist of thin metal strip is inserted into the tube to form a strong rotating flow and enhance heat transfer. If it can contact the tube wall closely, it can still act as a fin and expand the heat transfer surface. A large number of experimental studies have shown that the addition of inserts can significantly enhance the forced convection heat transfer, but also cause problems such as increased flow resistance, channel blockage and scaling. The insert should be used along the whole process of the pipeline to maintain the enhanced heat transfer in the whole process. In addition, when choosing the form of insert, the enhancement of heat transfer under small resistance should be considered.
Add rotating flow device
The centrifugal force of rotating flow will make the fluid produce secondary circulation, which will strengthen the heat transfer. In addition to their own characteristics, some of the above-mentioned inserts, such as twist iron and metal spiral wire, can also produce rotational flow. In this paper, we would like to mention some special components or devices which can produce rotational flow. For example, vortex generator, it can make the fluid under a certain pressure in the tangent direction into the tube for violent rotary movement. The results show that the extent of heat transfer enhancement is related to reynolds number. At a certain heat source temperature, the convective heat transfer coefficient increases with re value, and reaches a certain maximum value and then decreases. In application, the actual re value should be controlled close to the critical re value when the convective heat transfer coefficient reaches the maximum, so as to make full use of the effect of rotating flow. In addition to fluid rotation, there are also cases of heat transfer surface rotation. When the pipeline rotates around different axes, the secondary circulation generated by centrifugal force, shear stress, gravity and buoyancy can promote heat transfer enhancement. The enhancement effect of pipe rotation on laminar heat release is significant, but not obvious in turbulent flow. The experimental results of subcooled boiling and large space boiling show that the boiling heat transfer coefficient or critical heat load can be increased for the pipe with spiral inclined surface and tangential groove vortex generator.
Relying on external energy
In general, there are three measures:
- ① To vibrate the heat transfer surface or fluid by mechanical or electrical means, or mix the fluid well by stirring. The experimental results show that vibration has certain effect on free flow heat transfer and forced flow heat transfer. The effect of boiling heat transfer is not obvious, but the critical heat load can be significantly increased when the fluid vibrates in large space. It is difficult to apply this method to large heat exchanger. It is widely used, especially for high viscosity fluid.
- ② The fluid is compressed and expanded alternately by applying sound wave or ultrasonic wave to the fluid, so as to increase pulsation and enhance heat transfer. The results show that for liquid or gas, the acoustic effect is obvious only when it is in laminar or transitional flow. For large space bubble boiling, the effect is very small, but for transition boiling or film boiling, the heat transfer is improved significantly. It has a certain effect on condensation heat transfer and free flow heat transfer. In the application of acoustic strengthening measures, attention should be paid to solving the problem of how to transmit acoustic vibration or ultrasonic vibration to the interior of heat exchanger more effectively.
- ③ Electromagnetic field. High voltage is applied to the fluid involved in heat transfer to form a non-uniform radial electric field. Such electrostatic field can cause the mixing of dielectric fluid near the heat transfer surface, thus enhancing the convective heat transfer. The experimental results show that the enhancement effect on free flow heat transfer, film boiling heat transfer and condensation heat transfer is obvious. If the magnetic powder is added into the fluid, the magnetic field can enhance the heat transfer even at a large Re number. For example, adding magnetic powder into water or oil can increase the heat transfer coefficient by more than 50% under the action of magnetic field.
Physical properties of alternating fluids
The physical properties of the fluid have a great influence on the convective heat transfer coefficient. Generally, the fluid with larger thermal conductivity and volume specific heat has larger heat transfer coefficient. For example, the volume of water cooling is much smaller than that of air cooling in cooling equipment, because the α value between air and wall is in the range of 1 ~ 60W/(M2·℃), while that between water and wall is in the range of 200 ~ 12000W/(M2·℃). Another way to change some properties of fluid is to add some additives into the fluid. This is a new topic of heat transfer enhancement by additives formed in the past 20 or 30 years. The additive can be a solid or a liquid, which is combined with the heat exchange fluid to form gas-solid, liquid-solid, vapor-liquid and liquid-liquid mixed flow systems.
Change the heat exchange surface condition
The property, shape and size of the heat transfer surface have great influence on the convective heat transfer coefficient.
Increase wall roughness
Increasing the wall roughness is not only beneficial to the forced flow heat transfer in the tube, but also to the boiling and condensation heat transfer and the forced flow heat transfer outside the tube. The same roughness has different effect on heat transfer under different flow and heat transfer conditions. It should be considered in industrial application.
Change the shape and size of heat exchange surface
In order to increase the convective heat transfer coefficient, various special-shaped pipes and surface grooving can also be used, such as elliptical pipe, spiral pipe, corrugated pipe, variable cross-section pipe and longitudinal groove pipe, etc. Under the same cross-sectional area, the equivalent diameter of elliptical tube is smaller than that of circular tube, so the heat transfer coefficient is large. In addition to a slight increase in the heat transfer area of other special-shaped tubes, due to the change of surface shape, the direction and velocity of the fluid will change continuously, which will enhance the turbulence and reduce the thickness of the boundary layer. For the low rib spiral tube, it can also reduce the condensation film during the condensation heat transfer, which is beneficial to the condensation of organic working fluids (such as Freon). The micro fin tubes developed on the basis of low fin tubes are more conducive to the condensation heat transfer of organic media with low boiling point such as Freon, such as C tube in Japan and DAC tube in China. For vertical condensation, if the longitudinal groove tube is used, the condensate at the wave crest is pulled into the trough by the surface tension of the liquid, and the condensate film is formed at the wave crest, and the condensate is discharged from the trough, so the condensation heat transfer is enhanced.
Improve surface structure
A thin porous metal layer is formed on the surface of metal tube by sintering, EDM or cutting, which can enhance the boiling and condensation heat transfer. Such as: high heat flow tube of USA, E-tube of Japan, T-tube of Germany, DAE of China, etc. In addition, if a porous body is placed on the heating surface in boiling heat exchange liquid, the steam can be continuously removed through the porous heating surface, which is the so-called “suction” method, so as to improve the film boiling heat transfer.
In condensation heat transfer, a layer of material with low surface tension, such as polytetrafluoroethylene, can be coated on the heat transfer surface to cause bead condensation, which is conducive to increase the heat transfer coefficient. For boiling heat transfer, according to the physical properties of the heated liquid, the heating surface can be coated with a film of appropriate thickness to make it a non wetting surface, which can significantly improve the boiling heat transfer coefficient. In the utilization of solar energy, a thin layer of selective material is coated on the heat absorbing surface of the collector in order to improve its absorption rate of sunlight and reduce its emissivity, so as to enhance the absorption of radiant heat and reduce the radiation heat loss.
Source: China Stainless Steel Tubes Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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