6. Alternate applications benefit from offset strip fins

et al. [20] for the confirmation of the findings. Then, the velocity field around the fin during a complete period of oscillation is presented by contour plots for various time steps in the sequence of development. Moreover, unsteadiness is presented in the same manner. The second moment correlation of fluctuating velocity components and second moment of temperature velocity fluctuations are demonstrated. It is noted that heat transfer and moment transfer

Orientation of inlet and outlet headers plays a major role in performance of compact heat exchangers, especially in aerospace applications where the orientations are not straight. In order to understand the maldistribution at these components, Ismail et al. [21] studied these phenomena numerically. Three headers are developed to modify the distribution, which are combined with three offset strip fins and sixteen wavy fin geometries used in the heat exchangers. The computational results for wavy and offset strip fin structures are validated by analytical results and a good agreement is observed for j within –2% and for f within –9%, as it is illustrated in Figures 12 and 13. As for the headers, the flows are analyzed for either real (without modified headers) or ideal cases (with modified headers) and in all these conditions, the pressure drop is higher at real cases than at the ideal case. Among those, the biggest real

case-ideal case difference is observed for the heat exchanger with baffle plate by 34%.

Figure 13. The comparison of friction factor of different heat transfer surface types [21].

are dissimilar.

48 Heat Exchangers– Advanced Features and Applications

Even though the following applications are out of scope of this study, it is worth to note these in order to introduce the alternate ones to the readers who are not aware of. Solar air heater is one of these alternate systems that can transfer solar irradiant energy into airflow heat. These systems are typically used in space heating and food processing and they become popular for their cost-effectiveness and easy maintenance [22]. Like all other thermal systems, the development and research on these systems continue. In order to improve the performance of solar air heaters, researchers considered the use of offset strip fins [22–25] either numerically or experimentally. Results showed that, instantaneous thermal efficiency exceed 0.40 even at the lower air flow rates [22]. High thermal performance is obtained with low friction and as a result of all those power consumption of the fan descends [23]. In the laminar flow regime, the greatest temperature variation is recorded as 7°C at Re =479 [24]. On another study, the total amount of the collected heat is 612W/m2 for an incident heat flux of 900W/m<sup>2</sup> [25].

One of the other applications that uses offset strip fin is oil cooled heat exchangers where the need to improve heat transfer at the oil side is vital for the performance of the entire system. Due to the nature of the heat transfer medium, the fins are made up of steel instead of aluminium. The correlations for the system are derived from the experiments and compared with the experimental data; with regard to that, heat transfer is ±10% within the experiments with a mean deviation of 4.01%, while the friction factor can describe the experimental data within ±15% with a mean deviation 5.68% at low Re number [26]. Thermal hydraulic characteristics of offset strip fin are examined by considering the flow angle. Varying angles between 0 and 90° are observed and among those, the highest performance is observed at the angle 45° [27].

Ice slurry applications with its high transportation capacity by the latent heat, emerges as a promising alternate for the single phase coolant [28]. Pressure drop and heat transfer coefficient are investigated as common as in the OSF studies. But unlike the common approach, partial thermal resistance is examined as well. An empirical correlation for the Colburn j-factor is derived as well. The results revealing the instabilities of the flow may be seen due to total or partial blockages at isothermal conditions. In addition, a slight increase in the heat transfer rate and in the overall heat transfer coefficient is noted when the ice fraction increases at the flow.

Another two phase flow case is studied by some other researchers [29] for OSF heat exchangers. A vertical adiabatic channel OSF is tested for R113 (hydrofluorocarbon (trichloro-1,1,2-trifluoro-1,2,2-ethane), which is a refrigerant and now phased out in most of the developed countries) and not only pressure drop is measured but also two phase flow is visually photographed. A significant enhancement (40–50% higher) is observed for the two phase flow of R113 in OSF matrix than the conventional round tube.
