Nature-inspired double corrugated geometry for enhanced heat transfer

The PhD project “Improving the efficiency of heat pump and cooling technologies” is focused on research on efficiency improvements for heat pumps driven by the magnetocaloric effect (MCE). Magnetic refrigeration (MR) at room temperature is rather new; however a relatively mature technology in comparison with other caloric approaches. Nevertheless, the efficiency of the current state-of-the-art devices is not sufficient to bring this fascinating technology to commercialization. Therefore, many research efforts are devoted to improving the performance of magnetocaloric heat pumps through advances in the systems and materials. In this PhD project, major attention is paid to two components of magnetocaloric heat pumps, namely heat transfer and regenerator construction, which have significant importance to the performance of such devices.

The main working body in this kind of heat pumps is a solid regenerator of magnetocaloric material (MCM). The size and shape of the particles of MCMs as well as the number of layers in a regenerator play an important role in the performance of magnetocaloric heat pumps. Therefore, regenerators must be carefully constructed. Even a slight deviation of the Curie temperature between layers of MCMs leads to significant reduction of performance. In order to avoid the fatal failure, the designed regenerators are tested in smaller scale testing devices before using them in large-scale machines. This PhD thesis reports experimental results of eight regenerators, which were tested in a small-scale versatile testing device developed at Technical University of Denmark. Experiments on a regenerator with fifteen layers of MCM were conducted using a medium-scale active magnetic refrigeration (AMR) device at KTH Royal Institute of Technology. The tested regenerators varied in regenerator geometry, shape and size of particles as well as number of layers of MCM and amount of epoxy used for bonding particles. Experimental results revealed that epoxy bonding improves the mechanical integrity of the MCM in regenerators; however, it compromises their performance. Moreover, experiments proved that AMRs must be layered carefully in order not to jeopardize the performance. It was also found that a packed bed with spherical particles perform significantly better than similar beds with irregular particles from the efficiency standpoint.

The temperature difference established by the solid MCM regenerator is usually relatively small – around 25 – 40 K. Thus, it is challenging to assure an efficient heat transfer process in systems equipped with a conventional heat exchanger (HEX). Therefore, a novel solution for more efficient HEX is necessary. A nature-inspired tubular geometry for enhanced heat transfer is presented in this PhD thesis. The vascular counter-flow HEX, found in some fishes, was emulated and the obtained double corrugated tubes were numerically and experimentally analysed. The surface equations for these tubes were derived in a way that enables effortless adjustments of the diameter, equivalent to a straight tube, aspect ratio of the radial axes and the corrugation period. Either a constant hydraulic diameter, Dh, or a constant cross-section area, Ac, is maintained at any point along the flow channel of the designed geometry. An ellipse and a super-ellipse were used as a base geometry for double corrugated tubes, periodically converging to a circle or a perfect super-ellipse, respectively.

Computational fluid dynamics (CFD) analysis was carried out for the designed double corrugated tubes. The boundary conditions for CFD were constant surface temperature, constant pressure drop; and fully hydraulically developed flow with constant properties. Moreover, the modelling was performed at low temperature difference between the inlet and the tube wall, considering the possible application where only small temperature difference is available, such as heat pumps, or underfloor heating system. The CFD results showed that ellipse-based double corrugated tubes with constant Dh are generally more efficient than the same type tubes with the supper ellipse- based, or any double corrugated tubes with constant Ac. Major findings of the CFD analysis were the following:

1. Thermal performance of double corrugated tubes increases with increasing Reynolds number or decrease of corrugation period.
2. Higher aspect ratio leads to better thermal performance, as well as, severe reduction of a flow rate (for constant pressure drop).
3. The global thermo-hydraulic performance, evaluated at constant pressure drop, increases with increasing corrugation period.

Seven ellipse-based double corrugated tubes that demonstrated the best performance according to CFD results were manufactured from an aluminium alloy using selective laser melting technology. Five double corrugated tubes with constant Dh and two with constant Ac were tested in a tube-in-shell counter-flow heat exchanger using potable water as the working fluid in a range of Reynold numbers from 1000 to 2500. Correlations for the Nusselt number and friction factor for each tube are proposed and the experimental results show that Nusselt number increases up to 500 % in corrugated tubes compared to a straight tube while the friction factor increases less than 17 times. The global thermo- hydraulic performance, evaluated for the same pumping power, of the double corrugated tubes is up to 160 % higher than for a straight tube. The performance of tested double corrugated tubes is compared to other state-of-the-art geometries. The experimental results confirmed numerical predictions and demonstrated that double corrugated tubes with constant Dh have significantly higher thermal performance compared to constant Ac. However, the latter tubes exhibit lower increase in f when comparing the same aspect ratio and corrugation period. The global thermo-hydraulic performance is higher for double corrugated tubes with smaller corrugation period and for tubes with constant Dh.

Finally, an efficient AMR should have a high heat transfer rate and low pressure drop across the flow channel. The double corrugated tubes also demonstrated that they have these characteristics. Therefore, several experimentally tested tubes were analysed in a one-dimensional AMR model as a pattern for porous regenerator structure of a solid AMR. The numerical results obtained for double corrugated tubes were compared to results of regenerators with packed spheres and cylindrical micro- channel matrix. It is demonstrated that double corrugated tubes provide nearly two times higher cooling power than beds with packed spheres at the same value of Coefficient of Performance (COP). In other words, double corrugated tubes with certain geometrical characteristics provide same cooling power at higher COP than packed sphere at the same operating conditions.