Your calculated surface heat transfer coefficient will appear here.
Understanding the Lewis Formula (Surface Heat Transfer Coefficient)
The Lewis formula, in its common application related to heat transfer, is often used to estimate the surface heat transfer coefficient (h), particularly in scenarios involving natural convection or when simplifying heat transfer calculations. While there isn't one single universally "Lewis Formula" in all of physics and engineering, a widely recognized correlation, especially in thermal radiation and convection analysis, relates the heat transfer rate to the temperature difference and surface properties. This calculator focuses on a simplified, commonly applied form to determine the convective heat transfer coefficient (h) based on empirical correlations or simplified principles.
The formula this calculator uses is derived from the fundamental equation for convective heat transfer, often expressed as:
Q = h * A * ΔT
Where:
Q is the heat transfer rate (in Watts, W).
h is the convective heat transfer coefficient (in Watts per square meter per Kelvin, W/(m²·K)). This is what we are calculating.
A is the surface area through which heat is transferred (in square meters, m²).
ΔT is the temperature difference between the surface and the fluid (in Kelvin, K, or Degrees Celsius, °C).
This calculator rearranges the formula to solve for h:
h = Q / (A * ΔT)
Note on Emissivity: While emissivity (ε) is a critical factor in radiative heat transfer, it does not directly appear in the simplified convective heat transfer formula above. If you were calculating total heat transfer involving both convection and radiation, emissivity would be used in the radiative component (Q_rad = ε * σ * A * (T_surface^4 - T_surroundings^4)). This calculator specifically isolates and calculates the convective heat transfer coefficient (h) based on the provided Q, A, and ΔT.
Use Cases:
Estimating heat loss or gain in buildings.
Designing heat exchangers.
Analyzing cooling systems (e.g., electronics, engines).
Thermal management in aerospace and automotive applications.
Simplifying complex thermal simulations.
Example Calculation:
Let's say a surface is losing heat at a rate (Q) of 750 W to the surrounding air. The surface area (A) is 2.0 m², and the temperature difference (ΔT) between the surface and the air is 30 K.
Using the formula:
h = Q / (A * ΔT)
h = 750 W / (2.0 m² * 30 K)
h = 750 W / 60 m²·K
h = 12.5 W/(m²·K)
The calculated surface heat transfer coefficient is 12.5 W/(m²·K). This value indicates how effectively heat is transferred from the surface to the fluid via convection.
function calculateLewisFormula() {
var heatInput = document.getElementById("heatInput").value;
var temperatureDifference = document.getElementById("temperatureDifference").value;
var emissivity = document.getElementById("emissivity").value; // Included for completeness, though not used in this h calculation
var surfaceArea = document.getElementById("surfaceArea").value;
var resultDiv = document.getElementById("result");
// Input validation
if (heatInput === "" || temperatureDifference === "" || surfaceArea === "") {
resultDiv.innerHTML = "Please enter all required values.";
return;
}
var Q = parseFloat(heatInput);
var deltaT = parseFloat(temperatureDifference);
var A = parseFloat(surfaceArea);
// More robust validation for numbers
if (isNaN(Q) || isNaN(deltaT) || isNaN(A)) {
resultDiv.innerHTML = "Please enter valid numbers for all fields.";
return;
}
if (deltaT === 0) {
resultDiv.innerHTML = "Temperature difference cannot be zero for this calculation.";
return;
}
if (A === 0) {
resultDiv.innerHTML = "Surface area cannot be zero for this calculation.";
return;
}
// Calculate h using h = Q / (A * deltaT)
var h = Q / (A * deltaT);
// Display the result
resultDiv.innerHTML = "Calculated Heat Transfer Coefficient (h): " + h.toFixed(2) + " W/(m²·K)";
}