Heat transfer enhancement by using pulsating flows

Abstract

The paper presents a theoretical model of convective heat exchangers working with internal pulsating flows. It aims for a better physical understanding of the processes leading to a heat transfer enhancement inside these devices. When the frequency of the pulsation is increased, some geometries exhibit a maximum response, measured by its temperature rise, similar to those obtained in some dynamical resonant systems. The work explains the nature of this characteristic behavior and produces a simplified theoretical model that isolates the main physical features of the fluid dynamics involved. Two characteristic frequencies, measured by its Strouhal numbers, are theoretically found. The first one is associated with the spatial-averaged thermal response of the fluid near the wall and the second with the response of the velocity field. It is found, for a general device, that both Strouhal numbers and the maximum enhancement are mainly defined by the geometry of the device. Finally, the heat transfer enhancement of a straight channel, a backward facing step channel, and a two heated blocks inside an adiabatic channel are used to validate the model. Enhancements calculated with the present model are compared with the results reported in the scientific literature showing a good agreement for the tested cases.

Heat Transfer Enhancement for Weakly Oscillating Flows

“…In the discussion carried out by Benavides (2009), in order to overcome the problems derived from how the time-averaged temperature is defined (Guo & Suang, 1997), the heat flux was estimated as the time-averaged increment of the specific enthalpy along the device, i.e., as the value of s. Although, this approximation is interesting because the time-averaged outlet temperature is near the one that a thermocouple would measure in the exit port of an experimental arrangement, Eq. (78) shows that the constant part of s is not equivalent to the time-averaged heat flux under oscillating conditions.…”

“…(18) states that the heat transfer coefficient depends on the convention used for defining T P (Baehr & Stephan, 2006). This question is also important for oscillating flows, and hence it has been discussed in the literature (Faghri et al, 1979;Guo & Sung, 1997;Benavides, 2009). Note that Eq.…”

“…A negligible Prandtl number produces the higher velocity inside the thermal boundary layer. Although this extreme case is not representative of common fluids, it may be useful for obtaining an upper bound to the maximum heat transfer (Benavides, 2009 …”

“…These works show how the heat transfer reaches its maximum for a specific frequency of oscillation and decreases for both higher and lower values of frequency. This disparity of results motivated a theoretical study (Benavides, 2009), based on a second order expansion of the integral equations that governs the oscillating flow, which predicts the possibility of having heat transfer enhancement or deterioration as a function of the frequency of the oscillation. It is www.intechopen.com interesting to note that this feature is present even in the case of having flows where the amplitude of the velocity oscillation is small when it is compared with the time-averaged velocity inside the device.…”

“…This capability of oscillating flows to modulate the heat transfer process of some devices makes them interesting for some applications. Among the current devices that use oscillating flows are thermoacoustic engines and refrigerators (Backhaus & Swift, 2000;Gardner & Swift, 2003;Ueda et al, 2004), oscillatory flow reactors (Lee et al, 2001;Harvey et al, 2003), and, in general, any kind of heat exchangers with a periodical mass flow rate (Benavides, 2009). The theoretical characterization of the heat transfer process in such devices, where an oscillating flow is imposed over a stationary flow, is necessary either because it appears in a natural way such as in some kind of thermoacoustic devices (Benavides, 2006 or well because it is forced by pulsating the flow with vibrating or moving parts placed far enough in the upstream or downstream path (Wakeland & Keolian, 2004a, 2004b.…”