Browsing by Author "Alalaimi, M"

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Open Access
Counterflow heat exchanger with core and plenums at both ends
(International Journal of Heat and Mass Transfer, 2016-08-01) Bejan, A; Alalaimi, M; Lorente, S; Sabau, AS; Klett, JW
This paper illustrates the morphing of flow architecture toward greater performance in a counterflow heat exchanger. The architecture consists of two plenums with a core of counterflow channels between them. Each stream enters one plenum and then flows in a channel that travels the core and crosses the second plenum. The volume of the heat exchanger is fixed while the volume fraction occupied by each plenum is variable. Performance is driven by two objectives, simultaneously: low flow resistance and low thermal resistance. The analytical and numerical results show that the overall flow resistance is the lowest when the core is absent, and each plenum occupies half of the available volume and is oriented in counterflow with the other plenum. In this configuration, the thermal resistance also reaches its lowest value. These conclusions hold for fully developed laminar flow and turbulent flow through the core. The curve for effectiveness vs number of heat transfer units (Ntu) is steeper (when Ntu < 1) than the classical curves for counterflow and crossflow.
Open Access
Entrance-length dendritic plate heat exchangers
(International Journal of Heat and Mass Transfer, 2017-01-01) Bejan, A; Alalaimi, M; Sabau, AS; Lorente, S
© 2017 Elsevier Ltd. Here we explore the idea that the highest heat transfer rate between two fluids in a given volume is achieved when plate channel lengths are given by the thermal entrance length, i.e., when the thermal boundary layers meet at the exit of each channel. The overall design can be thought of an elemental construct of a dendritic heat exchanger, which consists of two tree-shaped streams arranged in cross flow. Every channel is as long as the thermal entrance length of the developing flow that resides in that channel. The results indicate that the overall design will change with the total volume and total number of channels. We found that the lengths of the surfaces swept in cross flow would have to decrease sizably as number of channels increases, while exhibiting mild decreases as total volume increases. The aspect ratio of each surface swept by fluid in cross flow should be approximately square, independent of total number of channels and volume. We also found that the minimum pumping power decreases sensibly as the total number of channels and the volume increase. The maximized heat transfer rate per unit volume increases sharply as the total volume decreases, in agreement with the natural evolution toward miniaturization in technology.
Open Access
The robustness of the permeability of constructal tree-shaped fissures
(International Journal of Heat and Mass Transfer, 2015-07-06) Alalaimi, M; Lorente, S; Wechsatol, W; Bejan, A
© 2015 Elsevier Ltd. Here we develop analytically the formulas for effective permeability in several configurations using the closed-form description of tree networks designed to provide flow access. The objective was to find the relation between the permeability and porosity of tree-shaped fissures. We found the effect of the fracture size on the permeability for fixed number of bifurcation and the results showed that the permeability of the fracture network increased rapidly with the size of the fracture. Next, we found a relation between the Reservoir Quality Index (RQI) and the porosity of the fracture. The results in this paper have been validated by comparison with experimental and numerical results. We show that the permeability formulas do not vary much from one tree design to the next, suggesting that similar formulas may apply to naturally fissured porous media with unknown precise details, which occur in natural reservoirs.
Open Access
Thermal coupling between a helical pipe and a conducting volume
(International Journal of Heat and Mass Transfer, 2015-01-01) Alalaimi, M; Lorente, S; Bejan, A
© 2014 Elsevier Ltd. All rights reserved. Here we document the effect of flow configuration on the heat transfer performance of a helically shaped pipe embedded in a cylindrical conducting volume. The helix is wrapped on an imaginary cylinder. Several configurations of helices with fixed volume of fluid are considered. We found the optimal spacings between the helical turns such that the volumetric heat transfer rate is maximal. Next, we extended the study by varying the volume (length) of the embedded pipe. We found that the optimized features of the heat transfer architecture are robust with respect to changes in several physical parameters. We compared the performance for both helical 3D and 2D designs. We found that the 2D designs offer greater heat transfer density than the 3D designs.