Research on Lightweighting of Plate-Fin Radiators

Amidst intensifying market competition, profit margins for plate-fin radiators are diminishing. Some conventional radiator types are even priced based on weight. To enhance profitability, companies are employing every possible method to reduce product weight as a means of cost reduction. The core of a radiator is its core structure, as shown in Figure 1, which accounts for up to 80% of the total weight. Therefore, weight reduction of the core becomes a crucial part of the design process for lightweighting. How, then, can the core be made lighter?

                                           Figure 1: Radiator Structure Diagram

The core, serving as the heart of the heat exchanger, is composed of fins, clad sheets, and seal bars. A basic unit is formed by placing a fin on a clad sheet, covering it with another clad sheet, and sealing the sides with seal bars, as illustrated in Figure 2. The core of a plate-fin heat exchanger is assembled from many such basic units. Based on this core structure, achieving weight reduction necessitates the lightweight design of its constituent components: seal bars, separator plates, and fins.

                                         Figure 2: Basic Unit of the Core Structure

Seal bars are located on both sides of each flow passage in a plate-fin heat exchanger. They come in various structural forms, with common widths ranging from 6 to 10 mm. A smaller width results in lower weight, but it can also impact the welding success rate. Currently, hollow seal bars are commonly used to reduce weight while maintaining an adequate welding area.

Clad sheets are the flat metal plates situated between two layers of fins, also known as double-sided clad sheets. They consist of a base alloy (typically aluminum-manganese alloy) coated on one or both sides with a brazing filler alloy. During the brazing process, this filler alloy melts, bonding the fins and the flat plates into an integrated whole. The filler alloy is generally an aluminum-silicon alloy containing 5~12% silicon, with a melting point typically about 40°C lower than that of the base material. Currently, the thickness of clad sheets has been reduced from 0.8 mm to 0.5 mm. Further reduction is limited by pressure resistance requirements and current process capabilities. In demanding applications like aerospace and the low-altitude economy, clad sheet thickness has been reduced to 0.45 mm or even 0.4 mm, though this imposes extremely high demands on the brazing process.

Fins are the most fundamental element of a plate-fin heat exchanger, primarily responsible for heat transfer. They are typically manufactured using either stamping or roll forming methods. In production, fins with lower heights and larger pitches are generally produced by roll forming (using a roll forming machine), while fins with greater heights and smaller pitches are typically produced by stamping (using a corrugating machine). Stamping offers lower production efficiency compared to the higher efficiency of roll forming. Common aluminum foil thicknesses used are 0.15 mm, 0.17 mm, and 0.2 mm.

Currently, radiators have largely achieved full aluminum construction. The aluminum foil used for fins is becoming progressively thinner, with a prevalent thickness of 0.17 mm currently, sometimes reaching 0.15 mm. In smaller core assemblies, foil thickness can even go down to 0.12 mm. As fin foil thickness decreases, its resistance to deformation weakens. Vacuum brazing involves temperatures very close to the material's melting point, making the material prone to softening and deformation. Core collapse and deformation leading to scrap can occur during the high-temperature brazing process, which is a significant cause of fin softening. Based on current vacuum brazing capabilities, the industry standard thickness is 0.17 mm, with 0.15 mm achievable for smaller cores. Experimental efforts are underway to further reduce fin thickness to 0.12 mm, but this is currently applied only in weight-critical aerospace applications and has not yet seen widespread industrial adoption.

Advanced manufacturing processes are crucial for enabling lightweight designs.

Vacuum Brazing Technology: This is a key process for manufacturing aluminum plate-fin radiators. It enables metallurgical bonding between fins and separator plates in a vacuum environment without the need for flux, resulting in a strong, clean, leak-free integrated structure.

Noclock Continuous Brazing: The shift from vacuum brazing to Noclock continuous brazing is being explored as a means to reduce costs.

Selecting different fin types (such as plain, serrated, or wavy) allows for balancing heat dissipation, flow velocity, and pressure drop. For instance, serrated fins enhance heat transfer by disrupting the thermal boundary layer. Some industry-leading ultra-thin all-aluminum radiators employ proprietary flat tube turbulence-promoting structures and densified fin configurations to improve heat exchange performance.

Technological development continues to focus on further weight reduction. For example, some patents propose designing V-shaped grooves and protrusions on short seal bars to reduce part weight while maintaining strength, thereby promoting radiator lightweighting.