Heat transfer enhancement is the process of increasing the effectiveness of heat exchangers. This can be achieved when the heat-transfer power of a given device is increased or when the pressure losses generated by the device are reduced. A variety of techniques can be applied to this effect, including generating strong secondary flows or increasing boundary-layer turbulence.
Principle
During the earliest attempts to enhance heat transfer, plain (or smooth) surfaces were used. This surface requires a special surface geometry able to provide higher h A {\displaystyle {hA}} values per unit surface area in comparison with a plain surface. The ratio of h A {\displaystyle {hA}} of an enhanced heat-transfer surface to the plain surface is called the Enhancement Ratio E h {\displaystyle E_{h}} . Thus,
E h = h A ( h A ) p . {\displaystyle E_{h}={\frac {hA}{(hA)_{p}}}.}
The heat-transfer rate for a two-fluid counterflow heat exchanger is given by
Q = U A Δ T m . {\displaystyle Q=UA\Delta T_{m}.}
In order to better illustrate the benefits of enhancement, the total length L of the tube is multiplied and divided in the equation
Q = U A L L Δ T m , {\displaystyle Q={\frac {UA}{L}}L\Delta T_{m},}
where L U A {\displaystyle {\frac {L}{UA}}} is the overall thermal resistance per unit tube length. It is given by
L U A = L η 1 h 1 A 1 + L t w k w A m + L η 2 h 2 A 2 . {\displaystyle {\frac {L}{UA}}={\frac {L}{\eta _{1}h_{1}A_{1}}}+{\frac {Lt_{w}}{k_{w}A_{m}}}+{\frac {L}{\eta _{2}h_{2}A_{2}}}.}
The subscripts 1 and 2 describe the two different fluids. The surface efficiency is represented by η {\displaystyle {\eta }} employing extended surfaces. One aspect to take into consideration is that the latter equation does not include any fouling resistances due to its simplicity, which can be important. In order to enhance the performance of the heat exchanger, the term UA/L must be increased. For achieving a reduced thermal resistance, the enhanced surface geometry may be used to increase one or both terms hA/L in relation to the plain surfaces, leading to a reduced thermal resistance per unit tube length, L/UA. This reduced term may be used to achieve one of the following three objectives:
Size reduction. Keeping the heat exchange rate Q {\displaystyle {Q}} constant, the length of the heat exchanger may be reduced, providing a heat exchanger of smaller proportions. Increased U A {\displaystyle {UA}} . Reduced Δ t m {\displaystyle {\Delta t_{m}}} : maintaining both Q {\displaystyle {Q}} and the length constant, Δ t m {\displaystyle {\Delta t_{m}}} can be reduced increasing thermodynamic efficiency, leading to reduced operation costs. Increased heat exchange: Increasing UA/L and keeping a constant length will lead to an increased Q {\displaystyle {Q}} for fixed fluid inlet temperature. Reduced pumping power for fixed heat duty. This will require smaller velocities of operation than the plain surface and an increased frontal area. Depending on the objectives for the design, any of the three different performance improvements can be used on an enhanced surface.
Internal flow
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