Thermal Calculations of Four-Row Plate-Fin and Tube Heat Exchanger Taking into Account Different Air-Side Correlations on Individual Rows of Tubes for Low Reynold Numbers

Abstract

Currently, when designing plate-fin and tube heat exchangers, only the average value of the heat transfer coefficient (HTC) is considered. However, each row of the heat exchanger (HEX) has different hydraulic–thermal characteristics. When the air velocity upstream of the HEX is lower than approximately 3 m/s, the exchanged heat flow rates at the first rows of tubes are higher than the average value for the entire HEX. The heat flow rate transferred in the first rows of tubes can reach up to 65% of the heat output of the entire exchanger. This article presents the method of determination of the individual correlations for the air-side Nusselt numbers on each row of tubes for a four-row finned HEX with continuous flat fins and round tubes in a staggered tube layout. The method was built based on CFD modelling using the numerical model of the designed HEX. Mass average temperatures for each row were simulated for over a dozen different airflow velocities from 0.3 m/s to 2.5 m/s. The correlations for the air-side Nusselt number on individual rows of tubes were determined using the least-squares method with a 95% confidence interval. The obtained correlations for the air-side Nusselt number on individual rows of tubes will enable the selection of the optimum number of tube rows for a given heat output of the HEX. The investment costs of the HEX can be reduced by decreasing the tube row number. Moreover, the operating costs of the HEX can also be lowered, as the air pressure losses on the HEX will be lower, which in turn enables the reduction in the air fan power.

Nusselt Numbers and Friction Factor Determination of Air-Side Finned Heat Exchangers – Individual Correlations of Each Row

“…The relative difference between the Nusselt number presented in this study and the Nusselt number correlation for Wang et al [17], is 19.42 % for 𝑅𝑒 𝑑 𝑜 = 1000 and 30.42 % for 𝑅𝑒 𝑑 𝑜 = 13,000 (Figure 4a). The relative difference between the Nusselt number presented in this study and the Nusselt number correlation for Marcinkowski et al [16] is -3.6 % for 𝑅𝑒 𝑑 𝑜 = 3,000, 0.33 % for 𝑅𝑒 𝑑 𝑜 = 1,000 and -1.58 % for 𝑅𝑒 𝑑 𝑜 = 13,000 (Fig. 4a).…”

“…4 shows the air-side Nusselt and the air-side D-W friction factor as a function of Reynolds number. Figure 4a compares the correlations determined by the current study with Wang et al [18], Wang et al [17] and Marcinkowski et al [16] in case of Nusselt number comparison. Table 3 presents geometry dimensions of the compared studies.…”

“…It can be noticed that all presented research have similar geometries. The relative differences between the current study and the other correlations were defined as: 𝑒 𝑝𝑎𝑟𝑎𝑚 𝑑𝑖𝑓𝑓𝐶𝑜𝑟𝑟 = 100(𝑝𝑎𝑟𝑎𝑚 𝑑𝑖𝑓𝑓𝐶𝑜𝑟𝑟 − 𝑝𝑎𝑟𝑎𝑚 𝑃𝑟𝑒𝑠𝑒𝑛𝑡 )/𝑝𝑎𝑟𝑎𝑚 𝑑𝑖𝑓𝑓𝐶𝑜𝑟𝑟 , where superscript diffCorr indicates that it is the value of a given study: Wang et al [18], Wang et al [17] and Marcinkowski et al [16]. The subscript param means the following parameters: Nu number and j-factor.…”

“…4a). The results of Marcinkowski et al [16] were performed by CFD modelling using Ansys CFX software. The current study was performed for the same mathematical model.…”

“…This method can use individual or average correlation of HTC, Nusselt number or Colburn factor. Marcinkowski et al [16] compared individual rows of tube air-side Nusselt numbers to average HTC in the entire FHE. This investigation showed that the heat flow rate transferred in the first rows of tubes can reach up to 65% of the heat output of the entire exchanger.…”