Composite Wall Experiment: Thermal Conductivity & Resistance

The composite wall experiment determines how heat conducts through several material layers in series and finds the overall thermal resistance and conductivity of the assembly. It applies Fourier’s law to layered walls — exactly how heat loss is calculated for furnace linings, building walls, and insulated equipment.

Aim of the experiment

To determine the total thermal resistance and overall thermal conductivity of a composite wall made of different materials in series.

Theory and formulas

For steady one-dimensional conduction through layers in series, the same heat flows through each layer and the thermal resistances add — just like resistors in series:

• Fourier’s law: Q = −kA (dT/dx)
• Thermal resistance of a layer: R = L / (kA)
• Total: Q = ΔT_overall / ΣR = A · ΔT_overall / Σ(L_i/k_i)

where L is layer thickness, k its thermal conductivity, and A the area. Measuring the heat input and the temperature drop across each layer gives each material’s conductivity and the overall U-value.

Apparatus required

• Composite wall apparatus — slabs of different materials (e.g. mild steel, brass, wood or asbestos, aluminium) clamped together
• Central electric heater between the slab stacks and a cooling plate
• Thermocouples at each interface and a wattmeter/heater control

Procedure

1. Clamp the slabs firmly together and switch on the central heater.
2. Allow the assembly to reach steady state, then record the heater input (Q) and the temperature at every interface.
3. Compute the temperature drop across each layer.
4. Calculate the thermal resistance of each layer and the total, then the overall conductivity.

Result

The total resistance equals the sum of the layer resistances, and the layer with the lowest conductivity (the insulator) shows the largest temperature drop — proving why insulation works.

Applications

Composite-wall heat flow governs furnace and boiler wall design, building insulation, cold-storage panels, and lagged equipment. It builds directly on the heat conduction experiment (linear & radial) and underpins exchanger design such as the double-pipe heat exchanger. Thermal conductivity is defined in the engineering lab glossary.

Frequently asked questions

Why do thermal resistances add in series for a composite wall?

Because the same heat flows through each layer in turn, the temperature drops add up, exactly like voltage drops across series resistors.

Which layer has the largest temperature drop?

The layer with the lowest thermal conductivity (the best insulator) has the highest resistance and therefore the largest temperature drop.

What is the U-value of a wall?

The U-value is the overall heat-transfer coefficient (the reciprocal of the total resistance per unit area); a lower U-value means better insulation.

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