Nuclear Engineering and Design 240 (2010) 3868–3877 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes A study of the effect of flow maldistribution on heat transfer performance in evaporators Julio C. Pacio ∗ , Carlos A. Dorao 1 Department of Energy and Process Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway article info Article history: Received 9 June 2010 Received in revised form 20 August 2010 Accepted 3 September 2010 abstract The performance of a heat exchanger with parallel channels can be seriously affected by flow maldistri- bution. In two-phase flow applications, such as evaporators, this situation may lead to local dryout and off-specification products. However, due to the complexity of describing this effect, it is normally not considered in the design process. Two models are proposed in this work for predicting the effect of maldistribution on performance. The first model neglects the interaction between channels, while the second one incorporates an equal pressure drop constraint. Both models analyze the geometry (shell-and-tube) in terms of radial layers. Here, the number of layers considered is irrelevant for the first model, while it affects the results of the second one. All results were compared to a homogeneous reference case, and reduction of performance was noticed in all cases. The first model predicts unrealistic pressure differences at the outlet. This situation is con- trolled by the second model, which predicts a secondary maldistribution to fit the constraint that further reduces the performance. When both models are compared, the first one underestimates the reduction in performance roughly by half. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Evaporators are widely used in industrial applications. In the power generation and thermal processing industries, they are used for vaporizing a heat-carrying fluid as in steam boilers. In addi- tion, since boiling yields large heat transfer rates, evaporators play a major role in refrigeration processes. The usual geometries (shell-and-tube, plate-type, tube-fin) use a parallel channels configuration, in order to accommodate a large heat-transfer area in a limited space. This allows the building of large capacity equipment within a reasonable size, but it also leads to problems related to flow maldistribution. In many scenarios the flow distribution can deviate from design conditions, which is usually homogeneous. The possible causes include poor header design or off-specifications operating condi- tions, and channel blockage or size reduction due to differential thermal expansion or fouling. A comprehensive review on causes of maldistribution can be found in Mueller and Chiou (1988). ∗ Corresponding author. Tel.: +47 73593662; fax: +47 73591640. E-mail addresses: julio.pacio@ntnu.no (J.C. Pacio), carlos.dorao@ntnu.no (C.A. Dorao). 1 Tel.: +47 73598462. Single-phase maldistribution has been widely studied, both from a header design perspective (Bassiouny and Martin, 1984a,b; Wang, 2008) and its effect on thermal performance (Lalot et al., 1999; Srihari et al., 2005). A reduction in heat duty (defined as the total heat transfered) is found in all cases as a consequence of mald- istribution. However, this effect is small (less than 5%) for most practical cases, with the exception of large-efficiency exchangers such as those used in cryogenic applications (Mueller and Chiou, 1988). For two-phase applications, research has been focused on the flow distribution in manifolds, both experimentally (Vist and Pettersen, 2004) and by numerical modeling (Ahmad et al., 2009; Ablanque et al., 2010). The thermal performance analysis was lim- ited to the effect of maldistribution on the single-phase streams, such as in condensers (Rabas, 1985; Bobbili et al., 2006) and evap- orators (Timoney and Foley, 1994; Aganda et al., 2000; Chen et al., 2005). As in the single-phase case, a deterioration of performance is predicted in most situations, but there is some experimental evidence of an increase in thermal performance on air-heated evap- orators (Timoney and Foley, 1994; Aganda et al., 2000). These models are based on energy balances, assuming a given flow distribution, usually based on the model given by Bassiouny and Martin (1984a,b). An average temperature difference and heat transfer coefficient are considered in the single-phase case (Mueller 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.09.004