Don’t understand basic heat transfer interview questions?
You won’t get the mechanical engineering job.
Think companies like Tesla, SpaceX, Blue Origin, Boeing, Raytheon, Northrop Grumman, etc.
Knowing basic heat transfer is necessary for both for a job interview and for general knowledge as a Mechanical Engineer.
You will not provide value if you don’t understand the concepts of basic heat transfer interview questions.
Engineering classes go very in depth, but in reality you only need to know the basics.
This post will aim to bridge the gap between what you learned in class and what you need to know to land your dream engineering job.
March 31, 2025 by Kazu Fujimoto
The following questions are basic concepts that you will go over in a heat transfer class.
Make sure you know these questions both so that you are ready for an interview and you also will need to know these terms if you want to be a good engineer.
Book a technical mock interview with Kazu to 10X chances of landing the engineering job.
Basic Heat Transfer Interview Questions
What are the three types of heat transfer and how do they differ?
The three types of heat transfer are conduction, convection, and radiation.
Conduction is the transfer of heat through a solid or stationary medium via molecular vibration, like heat moving through a metal rod.
Convection involves heat transfer by the movement of a fluid, such as air over a hot surface.
Radiation is the transfer of energy via electromagnetic waves, requiring no medium, like heat from the sun.
The key differences are the mechanisms: direct contact (conduction), fluid motion (convection), and wave propagation (radiation).
Can you explain Fourier’s law of conduction?
Fourier’s Law states that the heat flux (q) is proportional to the temperature gradient: q = -k * A * (dT/dx).
Here, q is the heat transfer rate (W), k is the thermal conductivity (W/m·K) measuring a material’s ability to conduct heat, A is the cross-sectional area (m²) perpendicular to heat flow, and dT/dx is the temperature gradient (K/m) driving the heat transfer.
The negative sign indicates heat flows from high to low temperature.
(Don’t need to go as far as to expressing units, in the US may use imperial units)
What is the difference between heat transfer coefficient and thermal conductivity?
Thermal conductivity (k) is a material property that quantifies how well a material conducts heat, measured in W/m·K.
The heat transfer coefficient (h), measured in W/m²·K, describes the rate of heat transfer between a surface and a fluid, influenced by fluid properties, flow type (laminar/turbulent), and geometry—not just the material.
For example, k is fixed for copper, but h varies with airspeed over a copper surface.
In short, thermal conductivity k for a material is constant, but heat transfer coefficient h can be variable given changes in air properties over a respective surface.
Critical Thinking and Application Based Questions
How would you calculate the overall heat transfer coefficient for a composite wall?
The overall heat transfer coefficient (U) is found using the total thermal resistance: 1/U = ΣR, where R is the resistance of each layer.
For a wall with conduction through layers and convection on both sides, R_total = (1/h₁) + (L₁/k₁) + (L₂/k₂) + … + (1/h₂).
Here, h₁ and h₂ are convective coefficients, L is thickness, and k is thermal conductivity of each layer.
Then, U = 1/(R_total * A), where A is the area. This accounts for all resistances in series.
What is the significance of the Nusselt number in convective heat transfer?
The Nusselt number (Nu = hL/k) is a dimensionless parameter that measures the enhancement of heat transfer due to convection compared to conduction alone. A Nu of 1 means pure conduction; higher values indicate convection dominates. It depends on flow conditions (e.g., Reynolds and Prandtl numbers) and geometry, helping engineers predict convective heat transfer rates.
Physically, Nu represents the ratio of convective heat transfer to conductive heat transfer over the length L. A higher Nu means convection is more dominant, while a lower Nu suggests conduction plays a larger role.
The Physical Interpretation of Nusselt Number
- Nu = 1: Heat transfer is purely conductive, as if the fluid were stagnant. For example, in a still fluid layer, heat moves only by molecular diffusion, and hL equals k.
- Nu > 1: Convection enhances heat transfer beyond conduction, due to fluid motion mixing warmer and cooler regions. This is common in engineering applications.
- Nu < 1: Rare, but possible in highly viscous or thin layers where convection is suppressed below conduction levels.
For instance, in turbulent pipe flow, Nu might be 100, meaning convection transfers heat 100 times more effectively than conduction alone across the pipe diameter.
Why is Nusselt Number so important?
The Nusselt number bridges theory and application. Since h is hard to measure directly, engineers use Nu correlations (derived from experiments or theory) to estimate h based on flow conditions and geometry. It’s critical for:
- Sizing equipment: A higher Nu means a smaller surface area (A) is needed for the same heat transfer rate (q = hAΔT).
- Optimizing designs: Adjusting flow (e.g., speed, turbulence) or geometry to increase Nu improves efficiency.
How Nu is Determined:
Nu depends on the flow regime, fluid properties, and geometry, often expressed via dimensionless groups:
- Reynolds Number (Re = ρvL/μ): Indicates laminar (smooth) vs. turbulent (chaotic) flow. Higher Re increases Nu due to better mixing.
- Prandtl Number (Pr = μc_p/k): Ratio of momentum diffusivity to thermal diffusivity, showing how fluid properties affect heat transfer. High Pr (e.g., oils) increases Nu in some cases.
- Geometry: Nu varies—e.g., Nu for a pipe differs from a flat plate.
What are some practical applications of Nusselt Number?
In a heat exchanger, a high Nu means efficient heat transfer, reducing size or cost. For a CPU heat sink, increasing Nu (via fans or fin design) keeps temperatures low. Conversely, in insulation design, a low Nu (still air) is desirable to minimize heat loss.
In summary, the Nusselt number is a subset of convective heat transfer analysis, taking into account fluid dynamics and thermal properties. It’s both a diagnostic tool and a design parameter, widely used across mechanical, chemical, and aerospace engineering.
What is Prandtl Number?
The Prandtl number (Pr) is a dimensionless parameter in heat transfer and fluid dynamics that characterizes the relative thickness of the momentum boundary layer to the thermal boundary layer in a fluid. It’s a fundamental property that helps engineers understand how momentum (velocity) and heat diffuse through a fluid under convective conditions
The Prandtl number is defined as:
Pr = ν/α
Where:
- ν (nu) is the kinematic viscosity of the fluid (m²/s), which measures how momentum diffuses due to viscosity. It’s given by ν = μ/ρ, where μ is dynamic viscosity (Pa·s) and ρ is density (kg/m³).
- α (alpha) is the thermal diffusivity (m²/s), which measures how heat diffuses through the fluid. It’s given by α = k/(ρcp), where k is thermal conductivity (W/m·K), ρ is density, and cp is specific heat capacity (J/kg·K).
Simply put, Pr = momentum diffusivity/thermal diffusivity
or if we substitute some of the variables above, we get
Pr = μcp/k
Physical Interpretation of Prandtl Number
The Prandtl number tells us how fast momentum spreads compared to heat in a fluid:
- **Pr < 1**: Heat diffuses faster than momentum (α > ν). The thermal boundary layer is thicker than the velocity boundary layer. Common in liquid metals like mercury (Pr ≈ 0.01–0.03)
- Pr = 1: Momentum and heat diffuse at the same rate (ν = α). The thermal and velocity boundary layers are roughly equal in thickness. This occurs approximately in gases like air (Pr ≈ 0.7–0.71)
- Pr > 1: Momentum diffuses faster than heat (ν > α). The velocity boundary layer is thicker than the thermal boundary layer. Typical for viscous fluids like water (Pr ≈ 7 at 20°C) or oils (Pr ≈ 100–1000)
This ratio influences how heat transfer behaves in convection—whether the fluid’s motion or its thermal properties dominate the process.
What is the relationship between Prandtl number and Nusslet Number?
The Prandtl number is critical in convective heat transfer because it affects the Nusselt number (Nu = hL/k), which quantifies convection efficiency. Nu correlations often include Pr to account for fluid properties:
- In laminar flow (e.g., over a flat plate), Nu ∝ Pr^(1/3). A higher Pr increases Nu, enhancing convective heat transfer because the thinner thermal boundary layer steepens the temperature gradient.
- In turbulent flow (e.g., Dittus-Boelter correlation, Nu = 0.023Re^0.8Pr^n), Pr^n (n = 0.4 for heating, 0.3 for cooling) reflects its role in turbulent mixing of heat and momentum.
As an example, oils with high Pr need more vigorous flow (higher Re) to achieve the same Nu as water, due to slower heat diffusion.
Calculate Prandtl Number for the following fluids:
Water at 20 deg C
Water at 20 deg Celsius will yield the following:
- μ = 0.001 Pa·s (dynamic viscosity)
- cp = 4180 J/kg·K (specific heat)
- k = 0.6 W/m·K (thermal conductivity)
As a result,
Pr = μcp/k
Pr = (.001) x (4180) / (0.6)
Pr = ~6.97
Air at 20 deg C
Air at 20 deg Celsius will yield the following:
- μ = 1.81×10⁻⁵ Pa·s
- cp = 1005 J/kg·K
- k = 0.026 W/m·K
As a result,
Pr = μcp/k
Pr = (1.81×10⁻⁵) x (1005) / (.026)
Pr = ~0.7
Water’s higher Pr reflects its greater viscosity and heat capacity relative to conductivity, while air’s near-unity Pr shows balanced diffusion.
What are some practical applications of Prandtl number?
- Heat Exchangers: Pr helps predict how fluids like water vs. oil perform. High-Pr fluids need larger surface areas or higher flow rates for efficient heat transfer.
- Aerodynamics: For air (Pr ≈ 0.7), thermal and velocity boundary layers align closely, simplifying aircraft wing cooling models.
- Material Processing: In molten metals (low Pr), rapid heat diffusion affects solidification rates in casting.
Describe a scenario where radiation heat transfer dominates over conduction and convection.
In space, radiation dominates because there’s no medium for conduction or convection (vacuum). For example, a satellite’s surface loses heat to space via radiation, proportional to T⁴ (Stefan-Boltzmann Law), while conduction is limited to its structure and convection is absent. High-temperature furnaces also rely heavily on radiation due to large temperature differences.
Practical and Application Based Heat Transfer Questions
How would you design a heat exchanger to maximize heat transfer efficiency?
I’d maximize surface area with fins or tubes, use a counterflow arrangement for a higher temperature gradient, select materials with high thermal conductivity (e.g., copper), and increase fluid velocity to boost the convective coefficient.
What is more efficient, parallel flow or counter flow in a heat exchanger, and why?
In a heat exchanger, counterflow (also called countercurrent flow) is generally more efficient than parallel flow(or cocurrent flow) for transferring heat between two fluids. This efficiency stems from the temperature distribution and the resulting driving force for heat transfer.
The key difference lies in the temperature gradient along the heat exchanger, which drives heat transfer (q = UAΔTm, where ΔTm is the mean temperature difference). Counterflow maintains a more consistent and favorable ΔTm, leading to better performance.
Counterflow is more efficient because it maintains a higher, more uniform temperature difference, leading to greater heat transfer and effectiveness. Its superiority is rooted in thermodynamics and is quantifiable via LMTD or ε-NTU methods. However, design trade-offs (cost, complexity, stress) might occasionally favor parallel flow.
See the math: Parallel vs counter flow in heat exchanger
What factors affect the rate of heat transfer in a finned surface, and how would you optimize it?
Key factors are fin length, thickness, spacing, material conductivity, and convective coefficient (h). Longer, thinner fins increase area but may reduce efficiency if too long (temperature drops). I’d optimize by choosing a high-k material (e.g., aluminum), spacing fins to avoid flow interference, and sizing them using the appropriate fin efficiency equations to iterate and balance heat transfer and cost.
If you’re given a pipe with hot fluid flowing inside and cold air outside, how would you estimate the heat loss?
I’d calculate it step-by-step:
(1) Inner convection (q = h₁A(T_fluid – T_inner_wall))
(2) Conduction through the pipe wall (q = kA(T_inner_wall – T_outer_wall)/L, using log mean area for cylindrical shape
(3) Outer convection (q = h₂A(T_outer_wall – T_air)).
Total resistance is 1/h₁A + ln(r₂/r₁)/(2πkL) + 1/h₂A.
Heat loss q = ΔT/R_total. Insulation could reduce losses if needed.
Behavioral Based Questions
Tell me about a time you worked on a heat transfer-related project. What challenges did you face?
In a university project, I designed a heat sink for an electronics board. The challenge was limited space, so I modeled fin spacing in CFD software to optimize convection while avoiding airflow blockage. Iterating designs took time, but we improved cooling by 20%, teaching me the value of simulation and testing.
How would you experimentally measure the thermal conductivity of a material?
I’d use the guarded hot plate method. Place the sample between a heated plate (controlled temperature) and a cold plate, with guard heaters to minimize edge losses. Measure the heat flux (q) and temperature difference (ΔT) across the sample thickness (L). Then, k = qL/(AΔT). I’d ensure steady-state conditions and calibrate instruments for accuracy.
Tip: Behavioral Based Interview Questions should be answered using the STAR format when possible
Closing Thoughts on Heat Transfer Interview Questions
Heat Transfer is a very fundamental engineering class that will stay throughout your entire career as an engineer.
There’s so much to cover from convection, conduction, radiation, to heat exchangers, to dimensionless parameters, and various heat transfer coefficients.
Use these basic heat transfer interview questions as a guideline to prepare yourself.
Don’t forget to check out our career services as we provide resumes, cover letters, interviews, and LinkedIn optimization for those specializing in engineering careers.
Book a technical mock interview with Kazu to 10X chances of landing the engineering job.
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About the author
Kazuyoshi Fujimoto, PE
Founder | Engineering Career Coach | Principal Mechanical Engineer
Kazu oversees all of ultmeche’s engineering services. He provides consulting such as resume reviews, rewrites, mock interviews, and all services career related. Additionally, Kazu performs consulting work regarding Oil & Gas, Automotive, and Aerospace & Defense. Kazu is licensed as a professional engineer in the state of California and has 9+ years of experience in Oil & Gas, Automotive, and Aerospace & Defense.