Return Bend Groove Geometry for Stable Evaporation in Fin-Tube Heat Exchangers
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Solution Overview
Problem
Conventional fin-and-tube heat exchangers face issues with refrigerant flow instability and evaporative performance due to swirling flow disruptions, pressure losses, and uneven refrigerant film thickness, particularly when using hydrofluorocarbon refrigerants like R410A, which affects heat transfer efficiency and evaporative performance.
Innovation Solution
The design incorporates a return bend tube with first grooves and a hairpin tube with spiral-shaped second grooves, optimized groove pitch and cross-sectional area ratios, and specific groove lead angles to maintain a stable 'annular flow' and reduce refrigerant film disturbances, along with a branched refrigerant flow channel and the use of a heat-resistant copper alloy for the return bend tube to enhance evaporative performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If grooves are formed inside the return bend tube to maintain swirling flow, then evaporative performance is improved, but the tube strength is reduced and manufacturing complexity increases
Solution Approach 1:
The patent applies local quality by forming grooves only in specific locations (return bend tube and hairpin tube inner surfaces) rather than uniformly across all surfaces. The grooves have specific dimensions (depth 0.05-0.2 times the inner diameter, pitch 0.3-0.6 times the inner diameter) optimized for local flow control while preserving overall tube strength. This localized modification allows the tube to maintain swirling flow where needed without compromising structural integrity throughout the entire tube.
2Productivity
If grooves are formed inside tubes to create swirling flow, then heat transfer efficiency is improved, but pressure loss increases
Solution Approach 1:
The patent optimizes groove parameters (depth, pitch, width) to balance heat transfer enhancement with pressure loss reduction. The groove depth is set at 0.05-0.2 times the inner diameter and pitch at 0.3-0.6 times the inner diameter, creating sufficient turbulence for enhanced heat transfer while avoiding excessive pressure drop. The groove cross-sectional area ratio between return bend tube and hairpin tube is controlled at 0.5-2.0 to maintain flow stability.
Solution Approach 2:
The grooves create dynamic swirling flow that adapts to the refrigerant flow conditions. The spiral geometry of grooves in the hairpin tube and the specific pitch in the return bend tube generate rotational flow that enhances heat transfer coefficient dynamically throughout the tube length, maintaining efficiency while controlling pressure loss through optimized geometric parameters.
3Ease of manufacture
If the return bend tube has a smooth inner surface, then manufacturing is easier, but swirling flow cannot be maintained and evaporative performance decreases
Solution Approach 1:
The patent applies local quality by forming grooves only in specific locations (return bend tube and hairpin tube inner surfaces) rather than uniformly across all surfaces. The grooves have specific dimensions (depth 0.05-0.2 times the inner diameter, pitch 0.3-0.6 times the inner diameter) optimized for local flow control while preserving overall tube strength. This localized modification allows the tube to maintain swirling flow where needed without compromising structural integrity throughout the entire tube.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration stabilizes the refrigerant flow, reduces pressure losses, and enhances evaporative performance by maintaining a uniform refrigerant film thickness, leading to improved heat transfer efficiency and reduced evaporative performance losses.
Implementation Method 1
the refrigerant flowing through the hairpin tubes develops a swirling flow along the grooves formed on the tube inner surface
Implementation Method 2
fin-and-tube heat exchanger in which a refrigerant such as a Freon-type refrigerant and a natural refrigerant flows inside tubes and a plurality of fins formed of aluminum or the like are arranged on the outer face of the tubes
Implementation Method 3
after inflow into the next hairpin tube, some time is lost until swirling flow is created again in the refrigerant, while there occurs droplet (refrigerant film) splashing at the bent portion of the return bend tube, which destabilizes the flow of the liquid film
Data Source
Figure 1
Figure 2
Figure 3(a)~3(c)
AI summary
The invention provides a fin-and-tube heat exchanger using a return bend tube that allows further enhancement of the evaporative performance of the heat exchanger. The fin-and-tube heat exchanger, where a refrigerant is supplied inside tubing, has a hairpin tube portion where a plurality of hairpin tubes are arranged, a return bend tube portion where there are arranged a plurality of return bend tubes joined to respective hairpin tube ends of the hairpin tube portion, and a fin portion comprising a plurality of fins arranged at a predetermined spacing on the outer surface of the hairpin tubes. The heat exchanger comprises first grooves formed on the tube inner surface of the return bend tube. A first groove pitch (P1) of the first grooves in a cross section perpendicular to a tube axis, and a second groove pitch (P2) of spiral-shaped second grooves formed on the inner surface of the hairpin tube in a cross section perpendicular to a tube axis, satisfy a groove pitch ratio (P1/P2) of 0.65 to 2.2, while a first groove cross-sectional area (S1) per groove of the first grooves in a cross section perpendicular to the tube axis, and a second groove cross-sectional area (S2) per groove of the second grooves in a cross section perpendicular to the tube axis satisfy a groove cross-sectional area ratio (S1/S2) of 0.3 to 3.6.