Experimental investigation and correlation development of shell-side thermohydraulic performance in a baffled microtube shell-and-tube heat exchanger with ultra-wide tube pitch

Microtube shell and tube heat exchangers (MTSTHXs) are highly compact and thermally efficient devices which are promising candidates for state-of-the-art heat management devices. On the other hand, due to the manufacturing limitations and the fouling concerns, ultra-wide tube pitches are usually adopted in such designs for which the thermohydraulic performance of the shell side is less known and there is no validation for any correlation developed for that particular case. The present study identifies a unique shell-side thermofluidic regime isolated cylinder flow which develops for a baffled microtube bundle when the tube pitch ratio is made higher to pt/do = 5.0. In this regime, the wake behind the tube does not interact with downstream rows, leading to the flow regime changing from wake-interactive cross-flow typical for compact bundles, pt/do = 1.25 – 1.5, to a weak-interaction regime that resembles an isolated cylinder flow. This change in the flow regime implies a change in the design concept where the goal is changed from generating wake turbulence to minimizing hydraulic friction, ensuring sufficient heat transfer. Experiments were carried out for a baffled MTSTHX with 58 SS316 microtubes (di = 0.7 mm, do = 1.0 mm, L = 160 mm), 60 o rotated triangular array, and four 30% cut segmental baffles at Re t = 1833 and Re s = 589 - 2944. The shell-side contribution accounts for 78 – 92% of total thermal resistance. Heat transfer coefficient on the shell side rises from 471 to 1548 W/m²K (+229%), while the overall coefficient increases from 433 to 1201 W/m²K (+177%). Six classical correlations for heat transfer on the shell side have errors from -25% to -57% (under-prediction) and from +15% to +47% (over-prediction). This shows that the design tools used for compact bundle do not work in this regime. Specific Nusselt number for this regime can be correlated by the equation Nus = 0.241 Res0.659 Prs1/3 (R² = 0.968, LOOCV MAPE = 6.9%). Shell-side pressure drop is less than 4 kPa only in 9% of the tube-side pressure drop, which confirms the hydraulic benefits of the regime.