Table of Contents
Understanding Heat Flux: The Foundation of Thermal Engineering
Heat flux is a flow of energy per unit area per unit time, measured in watts per square meter (W/m²). It has both a direction and a magnitude, making it a vector quantity. This fundamental parameter plays a critical role in thermal engineering, enabling engineers to analyze, design, and optimize systems involving heat exchange across diverse applications—from building insulation and HVAC systems to aerospace thermal management and electronic device cooling.
Heat flux is a fundamental concept in thermal analysis, pivotal in understanding how heat transfers in materials and systems across engineering and scientific disciplines, especially crucial in disciplines such as mechanical, aerospace, and civil engineering where thermal management is essential to system stability and safety. Understanding how to accurately calculate heat flux is essential for engineers working on thermal systems, as it directly impacts performance, safety, and energy efficiency.
This comprehensive guide provides engineers with detailed, step-by-step methods for calculating heat flux across all three modes of heat transfer: conduction, convection, and radiation. We’ll explore the underlying physics, practical calculation methods, real-world applications, measurement techniques, and important considerations for accurate thermal analysis.
The Physics Behind Heat Flux
Heat flux is a measure of the rate of heat energy transfer through a given surface per unit area, typically measured in watts per square meter (W/m²). The concept is rooted in the fundamental principle that heat naturally flows from regions of higher temperature to regions of lower temperature, driven by temperature gradients.
Thermal conduction is the diffusion of thermal energy (heat) within one material or between materials in contact, where the higher temperature object has molecules with more kinetic energy and collisions between molecules distributes this kinetic energy until an object has the same kinetic energy throughout. This microscopic view helps explain why temperature differences are the driving force behind all heat transfer phenomena.
Heat flux can occur through conduction, convection, or radiation, and each mode of transfer has its implications for how systems are designed. Understanding these three distinct mechanisms is crucial for selecting the appropriate calculation method and designing effective thermal management systems.
Heat Flux Calculation for Conduction
Conduction is the primary mode of heat transfer in solid materials and is governed by Fourier’s law of thermal conduction, one of the most important relationships in heat transfer analysis.
Fourier’s Law of Heat Conduction
Fourier’s law of thermal conduction states that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and the area, at right angles to that gradient, through which the heat flows. The mathematical expression is: Q = –k × A × (dT/dx), where Q is the rate of heat transfer (Watts), k is thermal conductivity (W/m·K), A is the area perpendicular to heat flow (m²), and dT/dx is the temperature gradient (K/m).
For heat flux specifically (heat transfer per unit area), the formula becomes:
q = -k × (dT/dx)
Where:
- q = heat flux (W/m²)
- k = thermal conductivity of the material (W/m·K)
- dT/dx = temperature gradient across the material (K/m)
The negative sign shows that heat flux moves from higher temperature regions to lower temperature regions. This mathematical convention ensures that heat flux is positive when flowing in the direction of decreasing temperature.
Thermal Conductivity: A Critical Material Property
The heat transfer characteristics of solid material are measured by a property called the thermal conductivity, k (or λ), measured in W/m·K, which measures a substance’s ability to transfer heat through a material by conduction. Fourier’s law applies to all matter, regardless of its state (solid, liquid, or gas), and the thermal conductivity of most liquids and solids varies with temperature, and for vapors, it also depends upon pressure.
Materials with higher thermal conductivity (k) conduct heat more efficiently—for example, diamond and metals like copper and silver are excellent conductors, while wood and air are poor conductors, and the law helps in designing insulation, cookware, cooling systems, and analyzing energy conservation in homes.
Common thermal conductivity values include:
- Copper: 385-401 W/m·K
- Aluminum: 205-237 W/m·K
- Stainless steel: 15-17 W/m·K
- Glass: 0.8-1.0 W/m·K
- Concrete: 0.8-1.4 W/m·K
- Wood: 0.1-0.2 W/m·K
- Air: 0.024-0.026 W/m·K
Simplified Conduction Formula for Steady-State Conditions
If heat flux is constant throughout a solid, then dT/dx can be replaced by ΔT/Δx, and this occurs in one-dimensional, steady-state heat flow—for example, if the two sides of a wall are held at two fixed temperatures, or the two ends of a laterally insulated wire are held at two fixed temperatures.
For practical engineering calculations with steady-state, one-dimensional conduction:
q = k × (ΔT / Δx)
Where:
- ΔT = temperature difference between the hot and cold surfaces (K or °C)
- Δx = thickness of the material (m)
Worked Example: Conduction Through a Wall
Calculate the heat flux through a glass window 1.5 m x 1.0 m in area and 3.0 mm thick if the temperatures at the inner and outer surfaces are 14.0°C and 13.0°C, respectively.
Given:
- Thermal conductivity of glass: k = 0.96 W/m·K
- Thickness: Δx = 3.0 mm = 0.003 m
- Temperature difference: ΔT = 14.0°C – 13.0°C = 1.0 K
Calculation:
q = k × (ΔT / Δx) = 0.96 × (1.0 / 0.003) = 320 W/m²
This result indicates that 320 watts of heat energy passes through each square meter of the glass window every second, demonstrating significant heat loss through the thin glass pane.
Heat Flux Calculation for Convection
Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid, and although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of conduction (heat diffusion) and advection (heat transfer by bulk fluid flow), and is usually the dominant form of heat transfer in liquids and gases.
Newton’s Law of Cooling
Convective heat flux is calculated using Newton’s law of cooling, which provides a simplified relationship between heat flux and temperature difference:
q = h × (Ts – T∞)
Where:
- q = convective heat flux (W/m²)
- h = convective heat transfer coefficient (W/m²·K)
- Ts = surface temperature (K or °C)
- T∞ = fluid temperature far from the surface (K or °C)
Newton’s Law of Cooling states that the heat flux q is proportional to the temperature difference between the object’s surface Ts and the fluid T∞, with the proportionality constant being the convective heat transfer coefficient h, and the value of h depends on the fluid properties, flow conditions, and surface characteristics.
Understanding the Convective Heat Transfer Coefficient
The heat transfer coefficient or film coefficient is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ΔT), used to calculate heat transfer between components of a system such as by convection between a fluid and a solid, with SI units in watts per square meter per kelvin (W/(m²K)).
Convective heat transfer coefficients depend on type of media, if it’s gas or liquid, and flow properties such as velocity, viscosity and other flow and temperature dependent properties, with typical values including: Free Convection – air, gases and dry vapors: 0.5 – 1000 (W/(m²K)); Free Convection – water and liquids: 50 – 3000 (W/(m²K)); Forced Convection – air, gases and dry vapors: 10 – 1000 (W/(m²K)); Forced Convection – water and liquids: 50 – 10000 (W/(m²K)); Forced Convection – liquid metals: 5000 – 40000 (W/(m²K)).
Types of Convection
In natural convection, an increase in temperature produces a reduction in density, which in turn causes fluid motion due to pressures and forces when the fluids of different densities are affected by gravity (or any g-force). Natural convection occurs without external forcing, driven purely by buoyancy effects.
Convection can be “forced” by movement of a fluid by means other than buoyancy forces (for example, a water pump in an automobile engine). Forced convection typically results in higher heat transfer coefficients due to increased fluid velocity and turbulence.
Worked Example: Convective Heat Transfer
A fluid flows over a plane surface 1 m by 1 m, with surface temperature of 50°C, fluid temperature of 20°C, and convective heat transfer coefficient of 2000 W/m²°C.
Calculation:
q = h × (Ts – T∞) = 2000 × (50 – 20) = 60,000 W/m²
This demonstrates the significant heat transfer that can occur with forced convection and high heat transfer coefficients.
Determining Heat Transfer Coefficients
The heat transfer coefficient is often calculated from the Nusselt number (a dimensionless number). The Nusselt number relates convective to conductive heat transfer and is determined through empirical correlations specific to the geometry and flow conditions.
Many correlations were developed by various authors to estimate the convective heat transfer coefficient in various cases including natural convection, forced convection for internal flow and forced convection for external flow, and these empirical correlations are presented for their particular geometry and flow conditions.
Heat Flux Calculation for Radiation
Radiative heat flux is a fundamental concept in thermodynamics and heat transfer, referring to the amount of thermal energy transferred in the form of electromagnetic radiation, where this energy transfer occurs between surfaces and environments without the involvement of a physical medium, and understanding radiative heat flux is crucial for applications in engineering, environmental science, and various other fields.
Stefan-Boltzmann Law
For an ideal absorber/emitter or black body, the Stefan–Boltzmann law states that the total energy radiated per unit surface area per unit time (also known as the radiant exitance) is directly proportional to the fourth power of the black body’s temperature, T.
The Stefan-Boltzmann law for radiative heat flux is:
q = ε × σ × T4
Where:
- q = radiative heat flux (W/m²)
- ε = emissivity of the surface (dimensionless, 0 ≤ ε ≤ 1)
- σ = Stefan-Boltzmann constant = 5.67 × 10-8 W/m²·K4
- T = absolute temperature of the surface (K)
The emissivity is generally between zero and one, with an emissivity of one corresponding to a black body. Real materials have emissivities less than 1, with highly polished metals having very low emissivities (0.02-0.1) and oxidized or painted surfaces having higher values (0.8-0.95).
Net Radiative Heat Transfer Between Surfaces
When calculating heat exchange between two surfaces at different temperatures, the net radiative heat flux is:
qnet = ε × σ × (Thot4 – Tcold4)
This accounts for the fact that both surfaces emit radiation, with the net heat transfer being the difference between emission and absorption.
Temperature Dependence of Radiation
The heat lost by radiation is described by the Stefan-Boltzmann radiation law, where the radiation loss depends on the fourth power of the temperature, which means that this mode of heat transfer is very important as temperature increases. This fourth-power relationship means radiation becomes increasingly dominant at high temperatures.
For example, doubling the absolute temperature increases radiative heat flux by a factor of 16 (24 = 16), making radiation the primary heat transfer mechanism in high-temperature applications such as furnaces, combustion chambers, and spacecraft thermal control.
Worked Example: Radiation from a Hot Surface
Calculate the radiative heat flux from a steel surface at 500°C with an emissivity of 0.85.
Given:
- Temperature: T = 500°C = 773 K
- Emissivity: ε = 0.85
- Stefan-Boltzmann constant: σ = 5.67 × 10-8 W/m²·K4
Calculation:
q = ε × σ × T4 = 0.85 × 5.67 × 10-8 × (773)4 = 17,300 W/m²
This demonstrates the substantial radiative heat loss from high-temperature surfaces.
Step-by-Step Heat Flux Calculation Procedure
Following a systematic approach ensures accurate heat flux calculations across all engineering applications. Here is a comprehensive procedure:
Step 1: Identify the Heat Transfer Mode
Determine whether heat transfer occurs primarily through conduction, convection, radiation, or a combination of these modes. Consider:
- Conduction: Heat transfer through solid materials or stationary fluids
- Convection: Heat transfer involving fluid motion (gases or liquids)
- Radiation: Heat transfer via electromagnetic waves, especially important at high temperatures or in vacuum
- Combined modes: Many real-world applications involve multiple mechanisms simultaneously
Step 2: Gather Required Data
Collect all necessary information for your specific calculation:
For Conduction:
- Material thermal conductivity (k)
- Material thickness or distance (Δx)
- Temperature difference (ΔT)
- Surface area (if calculating total heat transfer rate)
For Convection:
- Convective heat transfer coefficient (h)
- Surface temperature (Ts)
- Fluid temperature (T∞)
- Flow conditions (natural or forced convection)
- Fluid properties (if calculating h from correlations)
For Radiation:
- Surface emissivity (ε)
- Surface temperature(s) in Kelvin
- View factors (for complex geometries)
- Environmental temperature (for net radiation)
Step 3: Calculate Temperature Gradients or Differences
For conduction problems, determine the temperature gradient:
dT/dx ≈ ΔT / Δx (for steady-state, one-dimensional cases)
Ensure temperature units are consistent (typically Kelvin for radiation, Kelvin or Celsius for conduction and convection).
Step 4: Apply the Appropriate Formula
Select and apply the correct heat flux equation based on the heat transfer mode:
- Conduction: q = -k × (dT/dx) or q = k × (ΔT / Δx)
- Convection: q = h × (Ts – T∞)
- Radiation: q = ε × σ × T4 or qnet = ε × σ × (Thot4 – Tcold4)
Step 5: Verify Units and Dimensional Consistency
Ensure all quantities are in SI units:
- Heat flux: W/m²
- Thermal conductivity: W/m·K
- Heat transfer coefficient: W/m²·K
- Temperature: K (for radiation) or K/°C (for conduction/convection)
- Distance: m
Check that the final result has the correct units of W/m².
Step 6: Validate Results
Perform sanity checks on your calculated values:
- Does the magnitude seem reasonable for the application?
- Is the direction of heat flow correct (from hot to cold)?
- Do the results align with physical intuition and experience?
- For combined modes, is the total heat flux the sum of individual contributions?
Advanced Considerations in Heat Flux Calculations
Multi-Layer Conduction Systems
When heat is transported through a building wall, there are generally many different materials that affect the overall heat transfer, from the inside to the outside this could be for example: wallpaper → plaster → brickwork → insulation wall → render.
For composite walls with multiple layers, the total thermal resistance is the sum of individual resistances:
Rtotal = Δx1/k1 + Δx2/k2 + Δx3/k3 + …
The overall heat flux through the composite structure is:
q = ΔTtotal / Rtotal
This approach is analogous to electrical resistances in series, making it intuitive for engineers familiar with circuit analysis.
Combined Heat Transfer Modes
Many of the heat transfer processes encountered in nuclear facilities involve a combination of both conduction and convection—for example, heat transfer in a steam generator involves convection from the bulk of the reactor coolant to the steam generator inner tube surface, conduction through the tube wall, and convection from the outer tube surface to the secondary side fluid.
For combined modes, the overall heat transfer coefficient (U) incorporates all resistances:
1/U = 1/h1 + Δx/k + 1/h2
Where h1 and h2 are convective coefficients on each side, and Δx/k represents conductive resistance.
Temperature-Dependent Properties
In reality, the thermal conductivity is not a pure material constant, but depends on the temperature, and at large temperature differences, the thermal conductivity can therefore change relatively strongly over the thickness of the material, so in these cases, one has to use the mean value of the thermal conductivity.
For accurate calculations across large temperature ranges, use temperature-averaged properties or perform iterative calculations with property updates.
Non-Steady-State Conditions
The formulas presented above apply primarily to steady-state conditions where temperatures don’t change with time. When time is not involved in a heat transfer problem, we call it a steady-state thermal analysis. For transient (time-dependent) problems, more complex differential equations must be solved, often requiring numerical methods.
Surface Roughness and Contact Resistance
Real surfaces are not perfectly smooth, and contact between materials introduces additional thermal resistance. This contact resistance can significantly affect heat flux in applications involving mechanical joints, thermal interfaces in electronics, and bolted connections.
Material Heterogeneity
Many engineering materials are not homogeneous—composites, porous materials, and functionally graded materials have spatially varying properties. Regarding modern state-of-the-art applications of Fourier’s law, functionally graded materials (FGM) exhibit a spatial variation in material structure, which can be non-monotonic and even periodic, and this variation leads to corresponding variations in thermal properties.
Practical Applications of Heat Flux Calculations
Building Thermal Performance
In architectural engineering, heat flux calculations help in designing building envelopes with effective thermal insulation, reducing energy costs. Engineers use heat flux analysis to:
- Determine R-values and U-values for walls, roofs, and windows
- Optimize insulation thickness for cost-effective energy performance
- Identify thermal bridges and areas of excessive heat loss
- Comply with building energy codes and green building standards
In engineering practice, calculating and controlling heat flux is crucial—for example, in building engineering, controlling heat flux can improve the energy efficiency of buildings and reduce energy consumption, and by calculating the thermal conductivity of building materials and the temperature gradient under actual usage conditions, efficient insulation systems can be designed.
Electronic Thermal Management
In electronic devices, heat flux management is also an important issue, as electronic components generate significant heat during operation, which, if not effectively dissipated, can cause overheating and failure. Heat flux calculations enable:
- Design of heat sinks and cooling systems
- Thermal interface material selection
- Component placement optimization on circuit boards
- Reliability prediction and thermal stress analysis
Aerospace Applications
Thermal management is crucial in aerospace to protect structures and instruments from extreme temperatures encountered during high-speed flight or space missions. Heat flux analysis is essential for:
- Thermal protection system design for reentry vehicles
- Spacecraft radiator sizing
- Cryogenic propellant tank insulation
- Avionics cooling in high-performance aircraft
Industrial Process Control
Heat flux measurements and calculations are critical in numerous industrial processes:
- Furnace and boiler efficiency optimization
- Heat exchanger design and performance monitoring
- Chemical reactor thermal management
- Materials processing (casting, forging, heat treatment)
- Food processing and sterilization
Heat Flux Measurement Techniques
While calculations provide theoretical predictions, direct measurement of heat flux validates designs and enables real-time monitoring.
Heat Flux Sensors
A heat flux sensor is a transducer that generates an electrical signal proportional to the total heat rate applied to the surface of the sensor, the measured heat rate is divided by the surface area of the sensor to determine the heat flux, and the heat flux can have different origins—in principle, convective, radiative, as well as conductive heat can be measured.
The most common type of heat flux sensor is a differential temperature thermopile which operates on essentially the same principle as measuring temperature difference across known thermal resistance except it has the advantage in that the thermal resistance/conductivity does not need to be a known parameter, as these parameters do not have to be known since the heat flux sensor enables an in-situ measurement of the existing heat flux by using the Seebeck effect.
Types of Heat Flux Sensors
Heat flux sensors are known under different names, such as heat flux transducers, heat flux gauges, or heat flux plates, and some instruments are actually single-purpose heat flux sensors, like pyranometers for solar radiation measurement, while other heat flux sensors include Gardon gauges (also known as a circular-foil gauge), thin-film thermopiles, and Schmidt-Boelter gauges.
Thermopile Sensors: A heat flux sensor measures a small temperature difference across a thin layer of material, and this material typically employs a thermopile, which is an alternating pattern of two dissimilar metal alloys. These are the most common type for general-purpose applications.
Gardon Gauges: A Gardon type heat flux meter has a round foil of constantan soldered to a copper body, and when exposed to a heat irradiance, the heat flow is transmitted radially along the foil, creating a temperature gradient between the foil’s centre point and the edge, and this temperature difference is proportional to the absorbed heat flow and self-generates a linear voltage output.
Schmidt-Boelter Gauges: A Schmidt-Boelter type heat flux meter has a relatively thick thermopile mounted on a heat sink, the water-cooled body of the gauge, and the absorbed heat is conducted perpendicular to the absorber surface through the sensor into the heat sink.
Calibration and Accuracy
Differential thermopile heat flux sensors have to be calibrated in order to relate their output signals [μV] to heat flux values [W/(m²⋅K)], and once the heat flux sensor is calibrated it can then be used to directly measure heat flux without requiring the rarely known value of thermal resistance or thermal conductivity.
Critical to the use of a heat flux measurement technique is accurate calibration, and use of unmatched materials disturbs the local heat flux and also the local convective boundary layer, producing a potential error that must be compensated for, and the various techniques in common use for calibration are described.
Alternative Measurement Methods
Heat Flux Sensors are devices that directly measure the heat transfer across a surface, using thin, thermally conductive materials with embedded thermocouples or resistance temperature detectors (RTDs) to provide real-time data.
Calorimetry involves measuring the temperature change in a substance due to heat transfer, and by monitoring the mass, specific heat, and temperature change, engineers can calculate heat flux.
Infrared Thermography uses infrared cameras to detect surface temperatures and, combined with material properties, enables the calculation of heat flux.
Common Errors and How to Avoid Them
Unit Inconsistencies
One of the most common errors is mixing unit systems. Always ensure:
- Temperatures are in Kelvin for radiation calculations
- All lengths are in meters
- Thermal properties use consistent SI units
- Temperature differences can be in Kelvin or Celsius (they’re equivalent for ΔT)
Sign Convention Errors
The negative sign in Fourier’s law indicates direction. When calculating magnitude only, use absolute values. When direction matters (such as in energy balances), maintain proper sign conventions.
Neglecting Radiation at High Temperatures
At temperatures above approximately 100°C, radiation becomes increasingly significant. Neglecting radiative heat transfer in high-temperature applications can lead to substantial errors.
Assuming Constant Properties
Material properties vary with temperature. For large temperature differences, use average properties or account for temperature dependence explicitly.
Oversimplifying Geometry
One-dimensional heat transfer assumptions are valid only for specific geometries. Cylindrical and spherical geometries require modified equations that account for area changes with radius.
Ignoring Contact Resistance
In multi-layer systems with mechanical contacts, interface resistance can dominate. Include contact resistance in thermal resistance networks.
Computational Tools and Software
Modern engineering increasingly relies on computational tools for heat flux analysis:
Finite Element Analysis (FEA)
Heat flux can be estimated and analyzed much earlier in the design cycle of a product or part by leveraging the power of engineering simulation, particularly cloud-native simulation, and with simulation, you can visualize the heat flux and identify areas of concern that can inform your design decisions and enable you to optimize your design quickly.
Popular FEA software for thermal analysis includes ANSYS, COMSOL Multiphysics, Abaqus, and SimScale. These tools enable:
- Complex geometry modeling
- Transient thermal analysis
- Coupled thermal-structural analysis
- Optimization studies
- Visualization of temperature and heat flux distributions
Computational Fluid Dynamics (CFD)
For convection-dominated problems, CFD software solves the coupled fluid flow and heat transfer equations, providing detailed predictions of convective heat transfer coefficients and local heat flux distributions.
Spreadsheet Calculations
For simpler problems, spreadsheet software (Excel, Google Sheets) provides sufficient capability for heat flux calculations, parameter studies, and data analysis. Templates can be created for repetitive calculations.
Industry Standards and Best Practices
Several organizations provide standards and guidelines for heat flux measurement and calculation:
- ASTM Standards: ASTM C518 (thermal transmission properties), ASTM E1530 (thermal conductivity), ASTM C1046 (in-situ measurement of heat flux)
- ISO Standards: ISO 9869 (thermal insulation of building elements), ISO 8301 (thermal resistance and related properties)
- ASHRAE: Handbooks and standards for HVAC and building thermal performance
- IEEE: Standards for electronics cooling and thermal management
Following these standards ensures consistency, comparability, and regulatory compliance in engineering practice.
Future Trends in Heat Flux Analysis
The field of heat flux measurement and calculation continues to evolve:
Advanced Materials
Regarding modern state-of-the-art applications of Fourier’s law, functionally graded materials (FGM) and thermal metamaterials exhibit a spatial variation in material structure, which can be non-monotonic and even periodic, leading to corresponding variations in thermal properties, and FGMs can be found in various applications, from composites and porous materials optimized for mechanical properties to biomedical applications and even semiconductors.
Machine Learning and AI
To address the limitations associated with purely data-driven models, recent research has focused on integrating domain-specific physical knowledge into the AI/ML framework, leading to the development of Physics-Enhanced Machine Learning (PEML) approaches, and these hybrid methodologies aim to combine the predictive power of data-driven techniques with the rigor and consistency of established physical laws.
Miniaturized Sensors
With the growing demand for thermal management of electronic devices, cooling of high-precision instruments, and biological cryopreservation, heat flux measurement of complex surfaces and at ultralow temperatures has become highly imperative, and flexible and highly sensitive HFS that can operate at ultralow to high temperatures, ranging from −196 °C to 273 °C, with sensitivities reaching 11.21 μV/(W/m²) and 13.43 μV/(W/m²) are being developed.
Conclusion
Calculating heat flux accurately is a fundamental skill for engineers working in thermal systems design, energy efficiency, and thermal management. By understanding the three modes of heat transfer—conduction, convection, and radiation—and applying the appropriate formulas and methodologies, engineers can predict thermal performance, optimize designs, and ensure safe and efficient operation of thermal systems.
The step-by-step approach outlined in this guide provides a systematic framework for heat flux calculations across diverse applications. Remember to:
- Identify the dominant heat transfer mode(s)
- Gather accurate material properties and boundary conditions
- Apply the correct formulas with proper units
- Validate results against physical expectations
- Consider advanced factors like temperature-dependent properties and combined modes
- Utilize measurement techniques to verify calculations
As thermal challenges become increasingly complex—from nanoscale electronics to hypersonic vehicles—the importance of accurate heat flux analysis continues to grow. By mastering these fundamental calculation techniques and staying current with emerging tools and methods, engineers can effectively address the thermal management challenges of today and tomorrow.
For further exploration of heat flux and thermal engineering topics, consider visiting resources such as the Engineering ToolBox, Thermal Engineering Portal, ASHRAE, COMSOL Heat Transfer Resources, and Nuclear Power Heat Transfer Guide.