Flow/heat transfer characteristics in three heat exchanger geometries

A new study considered louvered fins as heat exchangers, as reported in the journal Energies. Their ability to exchange heat is due to the accessibility of a greater surface area, but they also cause the greatest pressure drops.

To study: Improved heat transfer by perforated and louvered fin heat exchangers. Image Credit: kkssr/Shutterstock.com

Heat exchanger equipment and its use

Heat exchangers transport thermal energy between two or more media, which can be fluid-fluid or fluid-gas systems. The heat transfer process is carefully considered when designing heat exchangers, which can include many types of heat transfer.

Heat exchangers are widely used in a variety of areas of the economy where controlled flow heating or cooling, controlled evaporation or monitored condensation is required, such as ventilation and air conditioning (HVAC) systems, industries of power generation, chemical processing, and manufacturing facilities.

(a) Image of perforated flat fin heat exchanger, (b) perforated hole arrangement, (c) louvered fin heat exchanger, (d) louvered fin shape.

(a) Photo of a perforated smooth fin heat exchanger, (b) layout of the punched holes, (vs) lamellar heat exchanger, (D) shaped like a slatted fin. Image credit: Atwieb, M., et al., Energies

Analysis and design of heat exchangers

Expertise and adherence to these throughout the design phase is essential to ensure proper and efficient operation. Methods for the analysis and design of heat exchangers have improved considerably over the years as a result of intensive studies in this area, and the focus is currently on the optimization of these systems.

The main objective of this research is to improve the heat transfer rate and minimize the pumping expenses, as well as the expenses related to the size and weight of the heat exchanger. In general, optimization techniques can be divided into two types: active and passive. An external force is used to drive heat transfer performance in the first category.

Variation of (Qmean [W]) versus Va for various heat exchangers;  (a) for a water flow of 0.12 m3/h, (b) for a water flow of 0.18 m3/h, (c) for a water flow of 0.24 m3/h , (d) for a water flow of 0.3 m3/h and (e ) for a water flow of 0.36 m3/h.

Variation of (Qmedium [W]) vs Va for various heat exchangers; (a) for water flow 0.12 m3/h, (b) for water flow 0.18 m3/h, (vs) for a water flow of 0.24 m3/h, (D) for a water flow of 0.3 m3/hand (and) for a water flow of 0.36 m3/h. Image credit: Atwieb, M., et al., Energies

Previous approach to detect efficiency

An experimental approach was created to determine the efficiency of different heat transfer technologies. The convection coefficients obtained were used to quantify the performance of the process.

The influence of fin (thickness, spacing) and tube (number of rows) factors on typical flow and heat transfer properties was also investigated. The researchers found that fin thickness and spacing had little impact on flow and heat transfer properties.

A prototype model was built to test heat transfer coefficients in steam formation and condensation processes using heat exchanger tubes. They extended the use of the underlying concept to a variety of convective heat transfer scenarios, noting that design engineers facing thermal difficulties would benefit from this.

Limits

Most studies have focused on the simple arrangement of punctures in single fins, with little information available on the complicated punctures of single fins. There is very little information available on louvered fins.

Perforations are used to improve passive heat transmission in heat exchangers. The influence of fin perforations on local and global performance measures is an important and ongoing area of ​​study that merits further investigation.

Additionally, most of the design equations provided are very narrow in range and do not account for various geometric features such as fin pitch, spacing, and existence of perforations.

Comparisons of predicted (by equations (9) and (10)) and experimental values ​​of (a) Colburn's j-factor and (b) Fanning's f-factor (c) United-finned Colburn's factor (d) the fanning factor with united fins.

Comparisons of predicted (by equations (9) and (10)) and experimental values ​​of (a) Colburn’s j-factor and (b) Ventilation factor f (vs) smooth-finned Colburn’s factor (D) Single fin fan factor. Image credit: Atwieb, M., et al., Energies

New research process

The new findings experimentally evaluate the steady state heat transfer and thermal performance of a variety of finned heat exchangers (simple, louvered and perforated fin). New techniques for the mechanical estimation of the Colburn factor and the Fanning friction factor as a function of the Reynolds number and the shape of the heat exchanger have been created and their estimated margins of error have been studied at the using experimental results. These equations should facilitate the construction and optimization of these heat exchanger configurations.

The result shows that Louvred Fin is among the best

Geometric layouts for multi-tube multi-fin heat exchangers have been created which are new. The configurations were created in order to undertake rigorous experimental analysis using three different geometric shapes of heat exchangers: smooth, perforated and lamella fin heat exchangers. During testing and review of pressure drop and heat transfer data, several significant results were obtained.

The louvered fins generated the maximum heat transfer rate for all intake air and water flow rates, and therefore speeds. This was attributed to an increase in surface area available for heat transmission. Compared to the other two designs, it also generated the greatest pressure losses.

Additionally, although the new perforated design created a slightly greater pressure drop than the single fin design due to the vortices formed by the perforations, it demonstrated improved heat transfer characteristics over the single fin designs. and louvered. At a fairly low water flow, this increase is quite high.

Advantage of this experience

The experimental data was then used to develop a set of innovative empirical equations for design optimization that can be used to anticipate the heat transfer and pressure drop characteristics of heat exchangers indicated by Colburn factors. and Fanning.

The equations were created as variables of the geometric characteristics of the heat exchangers, and we demonstrated that their performance is well within the acceptable margins of error of 15% compared to the experimental results.

Reference

Atwieb, M., et al. (2022). Improved heat transfer by perforated and louvered fin heat exchangers. Energies 2022, 15(2), 400; Posted: January 6, 2022 https://www.mdpi.com/1996-1073/15/2/400

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