**1. Introduction**

Microchannel heat exchangers are attractive due to their high ratio of heat transfer area to internal volume. In recent years, they have gained increased market shares in many refrigeration and air-conditioning applications as air-cooled condensers, because of better thermo-hydraulic performance and compactness compared with traditional finned tube heat exchangers. However, their use as evaporators in refrigeration systems is challenged by (1) water condensate retention and (2) poor refrigerant distribution. The first point is extremely important in frosting conditions, since any retained water after a defrost cycle will simply freeze again on the evaporator surface.

A recent development by SAPA (now Hydro) Precision Tubing called Web-MPE offers a compromise between compactness and condensate retention, claiming a reduction of 90% water retained compared with traditional microchannel design with louvered fins [1]. The new microchannel profile designs are made with specialized drain paths in between each microchannel port, which means that the coil becomes thicker in the airflow direction.

The aim of the current work is to provide airside heat transfer and pressure drop correlations that are applicable for the design of novel ammonia microchannel evaporators for industrial refrigeration systems, e.g., cold stores, blast freezers etc., where the evaporator operates in freezing conditions. Such ammonia evaporators are traditionally finned-tube evaporators and employ large tube diameters, large tube pitches, and large fin pitches resulting in large frosting periods (up to 24 h). The air velocity and the air throw length are high in these evaporators, hence, the tube circuitry is commonly inline to provide a low airside pressure drop. Fin types are typically limited to plain fins or wavy fins since other fin types (louver fins, offset fins) result in higher pressure drop and/or ice formation in the opening sections of the fins thus reducing their significance.

Charge minimization in ammonia refrigeration systems is pertinent due to safety restrictions associated to these systems. National authorities have implemented regulations to restrict the amount of charge in industrial refrigeration systems in many countries. Today the charge limit in Denmark is 5000 kg. Exceeding this limit leads to significant increase in cost of the plant, and installation, maintenance, and operation costs, due to increased safety precautions. It provides an incentive for academics, refrigeration engineers, and equipment manufacturers to target their research and development towards low-charge ammonia equipment, including the evaporator.

In the current paper, the backbone (airside heat transfer and pressure drop correlations) of a completely new type of low charge ammonia evaporator is developed by means of Computational Fluid Dynamics (CFD). The work is based on vertically oriented Web-MPE profiles with use of triangular plain fins. Compared to other fin geometries, such as plain fins or wavy fins, the triangular plain fins allow water to drain due to their vertical inclination. They result in less pressure drop compared with wavy fins, offset, and louvered fins. The correlations developed herein may be used by refrigeration engineers and researchers to design and optimize novel ultra-low charge ammonia evaporators. To the author's best knowledge, no previous investigations exist in the open literature considering the thermo-hydraulic characteristics of this novel microchannel design.

Computational Fluid Dynamics (CFD) has become a major tool in order to investigate the flow behavior and/or thermo-hydraulic performance inside compact heat exchangers having various fin types such as louvered fins [2–5], offset fins [6–8], wavy fins [9–11], helically wound finned-tube bundles [12,13], and plain fins [14,15]. The results of CFD simulations can be used to correlate the thermo-hydraulic performances, generally defined in terms of the Colburn j-factor and the Fanning f-factor. Chennu and Paturu [16] performed CFD simulations in order to develop air-side correlations for offset fins. They developed their correlations distinctively for laminar and turbulent regions. Ismail and Velraj [10] undertook similar work considering offset fins and wavy fins. Bacellar et al. [17] used CFD simulations to develop air-side correlations of a compact finned tube heat exchanger with staggered tube arrangement without fins. Damavandi et al. [11] expressed the air-side characteristics of a wavy fin-and-elliptical tube heat exchanger. They used neural network to express the j- and f-factors with the aim to optimize the geometry with using a j vs. f Pareto front. Deng [18] conducted CFD simulations using Large Eddy Simulations (LES) to improve correlations for flat tubes and louvered fins. Similarly, Sadeghianjahromi et al. [19] developed correlations for a finned tube heat exchanger with louvered fins, focusing on the effect of louver angle. The above references employ the effectiveness-NTU method or LMTD method with mass flow averaged temperatures to extract the j-factors. These methods incorporate the hydraulic and thermal entrance region. Other researchers assume fully developed flow and use stream-wise periodic boundary conditions, first proposed by Patankar et al. [20], for simplifying the computational domain. For example, Martinez-Espinosa et al. [21] made fully developed flow correlations for compact finned-tube heat exchangers having helically segmented finned tubes. Recent reviews on the performances of various compact heat exchanger designs can be found in Awais and Bhuiyan [22] and Qasem and Zubair [23], considering various fin types and both experimental and numerical data.

The present study investigates the new microchannel evaporator design. The objective is to establish heat transfer and pressure drop correlations in terms of Colburn j-factor and Fanning f-factor, for use in two-stream compact heat exchanger simulation and optimization codes. The correlations do not consider frosting or defrosting conditions, even though the microchannel profile has been developed herein to solve the problem of water condensate retention in evaporators during defrost. The aim of the work is rather to provide the scientific foundation that allows engineers and researchers

to design prototypes to be tested experimentally in frosting and defrosting conditions. Thus, the heat transfer and pressure drop in these conditions are subject for future work.

The paper is organized as follows: Section 2 describes the microchannel geometry, the CFD simulation design, modeling setup and verification, as well as data reduction methodology. Section 3 reports the results in terms of the correlations developed. In Section 4, the results and methodology are discussed. Finally, this is followed up by the conclusions in Section 5.

#### **2. Method**

#### *2.1. Geometry of the Microchannel Evaporator*

The microchannel evaporator is illustrated in Figure 1. It employs internal upward two-phase evaporating flow and external horizontal air crossflow. The triangular fins and drainage slits lead the water condensate downward through the evaporator during defrost.

**Figure 1.** Sketch of the new microchannel evaporator with condensate drainage paths (blue arrows).

The microchannels were extruded and punched aluminum profiles. The extruded profiles had fixed inner and outer tube dimensions and fixed longitudinal tube pitch (Figure 2a) corresponding to the extrusion counterpart. After the extrusion process, the profiles were punched to remove a large part of the tube fins bridging the tubes in order to accommodate water drainage during defrost or dehumidifying conditions (Figure 2b). The remaining tube fins bridging the tubes were assumed to have a negligible contribution to the airside heat transfer, and thus excluded in the CFD simulations. Furthermore, the triangular plain fins had a fin thickness of 0.1625 mm.

(**b**) Top view

**Figure 2.** Extruded aluminum profile before punching (**a**) and after punching (**b**).
