What Are The Factors That Affect The Heat Transfer Performance of Elliptical Tube Finned Tubes?

What Are The Factors That Affect The Heat Transfer Performance of Elliptical Tube Finned Tubes?

1. Structural Design: Determines Heat Transfer Area and Fluid Flow State

Structural design is a fundamental factor influencing heat transfer performance, directly determining the "contact area for heat exchange" and the "degree of turbulence during fluid flow." Key parameters include fin structure, elliptical tube cross-section, and tube-fin arrangement:

1. Fin Structural Parameters

Fins are the core component for expanding heat transfer area. Their structural details directly affect the "extended heat transfer area" and "fin-side thermal resistance" (the air/gas-side thermal resistance is usually the primary component of the total heat transfer resistance):

Fin Height (H):

The greater the fin height, the greater the expanded heat transfer area per unit length of the base tube (area is approximately proportional to height). However, excessively tall fins can lead to "fin efficiency reduction" (heat transfer from fin root to fin tip results in losses due to the fin's own thermal resistance. Fin efficiency can drop from 90% to below 70% for heights exceeding 15mm), thus reducing overall heat transfer performance. The optimal height is generally 8-12mm (suitable for air-side heat exchange). Fin Spacing (S):

The smaller the spacing, the greater the number of fins per unit volume, and the larger the heat transfer area. However, too small a spacing (<1.5mm) can lead to: ① a sharp increase in fluid flow resistance (airflow can easily become "blocked," increasing pressure drop by over 30%); ② dust and dirt can easily accumulate (especially in dusty air), creating "fouling thermal resistance" and reducing heat transfer efficiency by over 50% after long-term use. The optimal air-side spacing is generally 1.8-3mm, increasing to 3-5mm in dusty environments.

Fin Type (Straight Fins/Corrugated Fins/Louvred Fins):

Straight fins have a simple structure, but they are prone to forming a "laminar boundary layer" when fluid flows around them (the thicker the boundary layer, the greater the thermal resistance). Corrugated and louvered fins disrupt the boundary layer by "disturbing the airflow," increasing turbulence and improving the air-side heat transfer coefficient by 20%-40% (for example, the heat transfer coefficient of louvered fins is approximately 35% higher than that of straight fins), making them the preferred choice for high-efficiency heat exchange. Fin Thickness (δ):

The greater the thickness, the lower the thermal resistance of the fin itself (thermal resistance is inversely proportional to thickness). However, excessive thickness (>0.3mm) increases material costs and reduces effective flow area. Excessive thinness (<0.1mm) can easily cause fin deformation and poor vibration resistance. The optimal thickness is generally 0.15-0.2mm (aluminum fins) and 0.1-0.15mm (copper fins).

2. Elliptical Tube Cross-Section Parameters

The cross-sectional shape of an elliptical tube determines the "base tube heat transfer area" and "fluid flow resistance." The key parameters are the major axis (a), minor axis (b), and aspect ratio (a/b):

Aspect Ratio (a/b):

A larger aspect ratio (e.g., a/b = 3-4) reduces the "frontal area" of the elliptical tube (up to 25%-30% less than a circular tube of the same cross-sectional area). This results in a narrower vortex region during fluid flow, reducing flow resistance by 20%-30%. Furthermore, the flat cross-section extends the contact circumference between the fins and the base tube (15%-20% longer than a circular tube), ensuring a closer fit between the fins and the base tube, reducing "contact thermal resistance." However, an excessively large aspect ratio (>5) can reduce the base tube's strength and make it susceptible to deformation under vibration. Base tube wall thickness (t):

The smaller the wall thickness, the lower the base tube's own thermal resistance (thermal resistance is proportional to wall thickness). However, if the wall thickness is too thin (<0.5mm), it will affect pressure resistance (for example, high-pressure chemical applications require a wall thickness of ≥1mm). If the wall thickness is too thick (>2mm), the base tube's thermal resistance will increase significantly (for example, if the wall thickness of a stainless steel tube increases from 1mm to 2mm, the base tube's thermal resistance will double), offsetting the heat transfer advantages provided by the fins. 3. Tube-Fin Arrangement

The arrangement of the tube cluster directly affects the flow of fluid between the tubes. Common arrangements are "sequential" and "staggered":

Sequential: The tubes are aligned along the direction of fluid flow, resulting in smoother fluid flow and lower resistance (pressure drop is 15%-20% lower than staggered). However, the fluid is prone to forming a "wake dead zone" behind the tubes, resulting in lower turbulence and a 10%-25% lower heat transfer coefficient than staggered.

Staggered: The tubes are staggered along the direction of fluid flow. The fluid constantly changes direction as it flows around the tubes, resulting in higher turbulence and a 15%-30% higher heat transfer coefficient than sequential arrangement, but with higher flow resistance (pressure drop is 20%-30% higher).

Staggered arrangements are often chosen for high-efficiency heat exchange (such as air conditioning condensers), while sequential arrangements are recommended for low-resistance applications (such as large cooling towers). 2. Fluid Properties: Determining Convective Heat Transfer Intensity

The physical properties and flow state of the fluid (both the fluid inside and outside the tube) directly affect the "convective heat transfer coefficient" (h). The higher the convective heat transfer coefficient, the more efficient the heat exchange between the fluid and the tube wall, making it a key dynamic factor in heat transfer performance:

1. Fluid Physical Properties

Thermal Conductivity (λ):

The higher the fluid's thermal conductivity, the higher the convective heat transfer coefficient. (For example, when the fluid inside a tube is water, its thermal conductivity is approximately 0.6 W/(m・K), 23 times higher than that of air (0.026 W/(m・K)). The convective heat transfer coefficient on the water side can reach 1000-5000 W/(m²・K), while on the air side it is only 30-80 W/(m²・K)). Therefore, when the fluid inside a tube is liquid, the heat transfer performance is far superior to that of gas. Specific Heat Capacity (cₚ) and Density (ρ):

The greater the specific heat capacity and density, the greater the fluid's "heat capacity" (the more heat it can carry per unit volume of fluid), and the higher the heat transfer rate during convective heat transfer. For example, the specific heat capacity of water (4.2 kJ/(kg・K)) is 4.2 times that of air (1.0 kJ/(kg・K)), and its density is 800 times that of air. At the same flow rate, the "volume heat transfer coefficient" (ρ・cₚ・u, where u is the flow rate) on the water side is over 3000 times that on the air side.

Viscosity (μ):

The greater the viscosity, the greater the internal frictional resistance of the fluid during flow, the lower the turbulence, and the lower the convective heat transfer coefficient. For example, the viscosity of engine oil (approximately 0.1 Pa·s at room temperature) is 100 times that of water (0.001 Pa·s). At the same flow rate, the convective heat transfer coefficient on the oil side is only 1/10-1/20 of that on the water side.

2. Fluid Flow State

Flow Velocity (u):

Flow velocity is the core factor affecting the convective heat transfer coefficient. The higher the flow rate, the more intense the relative motion between the fluid and the pipe wall, and the thinner the boundary layer. The convective heat transfer coefficient is proportional to the 0.6-0.8 power of the flow velocity (for example, increasing the air flow velocity from 2 m/s to 4 m/s can increase the convective heat transfer coefficient by 60%-90%). However, excessively high flow rates (e.g., air flow rates > 8 m/s) lead to a sharp increase in flow resistance (pressure drop is proportional to the square of the flow velocity), increasing energy consumption. Therefore, a balance must be struck between "heat transfer efficiency" and "energy cost." Flow Pattern (Laminar/Turbulent):

In laminar flow (Reynolds number <2300), the fluid flows in layers along the tube wall. Heat is transferred primarily through molecular diffusion, resulting in high thermal resistance. In turbulent flow (Re>4000), strong internal turbulence occurs within the fluid, and heat is transferred through eddy mixing. The convective heat transfer coefficient is 3-5 times higher than that of laminar flow. The streamlined structure of the elliptical tube reduces the critical Reynolds number for turbulence (15%-20% lower than that of a circular tube), making it easier to develop turbulence and improving heat transfer efficiency. III. Material Selection: Determining the Thermal Resistance of the Solid Heat Conductive Path

Heat transfer in elliptical finned tubes requires a solid heat conduction path from the fluid inside the tube to the base tube to the fins to the fluid outside the tube. The thermal conductivity of the base tube and fin materials directly determines the "solid thermal resistance," a fundamental factor in ensuring heat transfer performance:

1. Base Tube Material

The base tube must meet both "high thermal conductivity" and "operating adaptability" (pressure resistance and corrosion resistance). Thermal conductivity varies significantly between different materials:

Copper (red copper): With a thermal conductivity of approximately 401 W/(m・K), it offers the best thermal conductivity and is suitable for medium- and low-pressure, non-corrosive environments (such as air conditioner evaporators/condensers). However, it is costly and exhibits poor corrosion resistance (susceptible to seawater, acids, and alkalis).

Aluminum (pure aluminum/aluminum alloy): With a thermal conductivity of approximately 237 W/(m・K), it is second only to copper in thermal conductivity. It is also low-cost and lightweight (its density is only 1/3 of copper). 1/3), suitable for household heat exchangers and automotive air conditioners, but with poor pressure resistance (not suitable for high-pressure conditions);

Stainless steel (304/316L): Thermal conductivity is approximately 16-19 W/(m・K), only 1/20 that of copper. Solid thermal resistance is high, but corrosion resistance is strong (suitable for acid, alkali, and high-temperature flue gas applications). Insufficient thermal conductivity requires increasing fin area (for example, stainless steel oval finned tubes in chemical applications require a 20%-30% larger fin area than copper tubes).

Titanium alloy: Thermal conductivity is approximately 21 W/(m・K). It offers extremely strong corrosion resistance (especially seawater resistance), but its cost is high, making it suitable only for extreme applications such as marine engineering and high-end chemical engineering. 2. Fin Material

Fins must be made of a material that combines high thermal conductivity with good ductility (easily processed). The mainstream choices are:

Aluminum (1060 pure aluminum): With a thermal conductivity of approximately 237 W/(m・K), it is low-cost and has good ductility (it can be processed into thin fins). It accounts for over 90% of fin materials and is suitable for most civilian applications.

Copper (T2 copper): With a thermal conductivity of approximately 401 W/(m・K), it offers excellent thermal conductivity, but is also high-cost and has slightly lower ductility (thin fins are prone to cracking). It is only used in high-end, high-efficiency heat exchange applications (such as aerospace heat exchangers).

Aluminized steel: Aluminum is infiltrated onto the surface of steel fins, forming an Al₂O₃ oxide film. This provides strong temperature resistance (capable of withstanding temperatures up to 800°C) and is suitable for boiler waste heat recovery and high-temperature flue gas heat exchange. However, its thermal conductivity is relatively low (approximately 45 W/(m・K)). IV. Operating Conditions: Determining the "Driving Force" and Stability of Heat Transfer

Operating conditions are the "external environment" of the heat transfer process, directly affecting the "temperature difference" (driving force) and "long-term stability" of heat transfer. Key parameters include temperature, pressure, and fouling deposition:

1. Temperature Conditions

Temperature Difference Between Hot and Cold Fluids (ΔT):

The heat transfer rate is proportional to the temperature difference (according to Fourier's law, Q = K·A·ΔT, where Q is the heat transfer rate). The greater the temperature difference, the stronger the driving force for heat transfer. However, excessive temperature differences (e.g., a temperature difference > 200°C between the high-temperature fluid inside the tube and the low-temperature air outside the tube) can result in: ① a large difference in the thermal expansion coefficients of the base tube and fins, generating thermal stress at the junction and potentially causing fin detachment; ② condensation/frost formation on the fin surface (when the air-side temperature is below the dew point), creating "phase change thermal resistance" and reducing heat transfer efficiency by 30%-50%. Fluid Temperature Range:

Excessively high temperatures (e.g., fluid in the tube > 400°C) will cause the material's thermal conductivity to decrease (e.g., aluminum's thermal conductivity drops 15% at 300°C compared to room temperature) and easily oxidize the fins (forming an oxide layer, increasing thermal resistance). Excessively low temperatures (e.g., air outside the tube < -10°C) will cause frost to form on the fin surface, increasing the thickness of the frost layer (the thermal resistance of a 1mm frost layer is equivalent to that of 20mm of copper), requiring a defrost function to maintain heat transfer performance.

2. Pressure Conditions

Tube Pressure:

Excessively high tube pressure (e.g., > 10MPa) requires thick-walled base tubes (e.g., stainless steel tubes with a wall thickness of ≥ 2mm), but this increases the base tube's thermal resistance. Excessively low pressure (e.g., vacuum conditions) will hinder boiling/evaporation of the fluid in the tube, reducing the convective heat transfer coefficient (e.g., the boiling heat transfer coefficient of water in a vacuum is only 1/5 of that at atmospheric pressure). External pipe pressure (air velocity/flow rate):

The external fluid pressure (e.g., fan pressure) determines the flow rate. Insufficient pressure results in low flow rates and a reduced convective heat transfer coefficient. Excessive pressure results in excessively high flow rates, increased resistance, and higher energy consumption. Therefore, an optimal flow rate must be matched (typically 2-5 m/s on the air side and 1-3 m/s on the water side). 3. Dirt Deposition

When dirt (such as dust, scale, and oil) accumulates on tube walls and fin surfaces, it forms "fouling thermal resistance (Rf)," a primary cause of heat transfer performance degradation over long-term use.

Air-side dirt: Examples include dust, catkins (in residential air conditioners), and dust (in industrial applications). A 1mm thick layer of dust has a thermal resistance of approximately 0.04m²/K/W, equivalent to 2-3 times the air-side convection thermal resistance, and can reduce the overall heat transfer coefficient by 30%-40%.

Water-side dirt: Examples include scale (calcium and magnesium ion deposits) and biological sludge (in circulating water applications). A 0.5mm thick layer of scale has a thermal resistance of approximately 0.02m²/K/W, reducing the water-side convection heat transfer coefficient by 20%-30%.

Therefore, in applications prone to scaling, select anti-scaling materials (such as nickel-plated copper tubes) or easy-to-clean structures (such as wide fin spacing) and clean them regularly. 5. Surface Condition: Impacts Fluid-Wall Contact Efficiency

The surface condition of the fins and base tube directly affects fluid-wall contact and phase change processes (such as condensation and boiling). Key factors include surface roughness and surface treatment:

1. Surface Roughness (Ra)

The smoother the surface (Ra < 0.8μm), the lower the fluid flow resistance and the less likely it is to accumulate dirt. Furthermore, under condensation conditions (such as air conditioning condensers), condensate on smooth surfaces tends to form "film condensation" (thickening the condensate film and increasing thermal resistance).

Rough surfaces or porous treatments (such as spraying a porous coating on the fin surface) promote "droplet condensation" (condensate sheds as droplets, preventing film formation). This can improve the condensation heat transfer coefficient by 50%-100%, making it suitable for condensation applications. 2. Surface Treatment

Hydrophilic Coating: Spraying a hydrophilic material (such as acrylic resin) on the fin surface allows condensed water to quickly diffuse into a water film and flow down, preventing the film from thickening (reducing phase change thermal resistance) and preventing dust from adhering (the water film can carry away dust). This process is commonly used on air conditioner evaporators and can improve heat transfer efficiency by 10%-15%.

Hydrophobic Coating: Spraying a hydrophobic material (such as polytetrafluoroethylene) on the fin surface causes condensed water/frost to fall off in the form of particles, preventing frost from sticking (commonly used on cold storage evaporators). This can reduce defrost time by 20%-30%.

Anti-corrosion coatings, such as hot-dip zinc coatings and ceramic coatings, are used in corrosive environments (such as marine salt spray and chemical acid and alkali environments). They prevent oxidation corrosion (the oxidation layer increases thermal resistance), extending service life while maintaining heat transfer performance.