Select working fluid, wick type, operating temperature, and pipe dimensions to compute capillary limit, effective thermal conductance, and max heat flux. Compare all four fluids side by side.
Design Parameters
Working fluid
Wick type
Operating temp T
°C
Pipe diameter D
mm
Pipe length L
m
Results
While paused, move the sliders to update the result instantly.
Live Operating Values
0
Heat transport Q [W]
0
k_eff [kW/m·K]
0
Vapor flow [g/s]
0
Temp drop ΔT [°C]
Working Cycle: Evaporation → Vapor → Condensation → Capillary Return
What exactly is the "capillary limit" this simulator calculates? I see it as the main output, but what does it physically mean?
🎓
Basically, it's the maximum heat the pipe can transfer before it dries out. The wick uses capillary action—like a paper towel soaking up water—to pump liquid from the cool end back to the hot end. If the heat load is too high, the pump can't keep up, the hot end dries out, and the pipe fails. Try selecting different "Wick types" above; you'll see the limit change dramatically because the wick's pumping power is different.
🙋
Wait, really? So the fluid choice matters too? Why would using ammonia versus water change the limit if the pipe is the same size?
🎓
Absolutely. Different fluids have different physical properties that are crucial for capillary pumping. For instance, ammonia remains useful in low-temperature ranges where water would freeze, but its lower surface tension ($\sigma$) reduces capillary pumping force. Water generally gives the larger heat-transport capacity in the overlapping temperature range. Change the "Working fluid" dropdown and watch the max heat flux update—it's a direct trade-off the simulator lets you explore.
🙋
So the operating temperature slider must be super important then. If I'm designing a system for a satellite in the cold of space versus a laptop CPU, how do I use this tool to pick the right temperature?
🎓
Great question. The temperature directly affects all the fluid properties in the equation. For a given fluid, there's an optimal temperature range where its surface tension and viscosity create the best pumping performance. A common case is using methanol around 50-70°C for electronics cooling. Slide the "Operating temp T" control and observe how the capillary limit peaks at a certain temperature for your chosen fluid—that's your design sweet spot.
Physical Model & Key Equations
The core performance metric is the capillary limit, which determines the maximum heat transfer rate before dryout. It balances the capillary pumping pressure against the sum of viscous pressure drops in the liquid and vapor flows. The most common simplified form for the capillary limit is:
$$Q_{\max}= \frac{2\sigma h_{fg}}{\mu_l r_{\rm eff}}\cdot \frac{\rho_l K A_w}{L}$$
Where:
$Q_{\max}$ = Maximum heat transfer (W)
$\sigma$ = Liquid surface tension (N/m)
$h_{fg}$ = Latent heat of vaporization (J/kg)
$\mu_l$ = Liquid dynamic viscosity (Pa·s)
$r_{\rm eff}$ = Effective pore radius of the wick (m)
$\rho_l$ = Liquid density (kg/m³)
$K$ = Wick permeability (m²)
$A_w$ = Cross-sectional area of the wick (m²)
$L$ = Total heat pipe length (m) – the "Pipe length L" in the simulator.
The effective pore radius ($r_{\rm eff}$) and permeability ($K$) are determined by the wick structure. This is why the simulator's "Wick type" parameter is so critical. For a simple screen wick, these can be approximated from the mesh size and wire diameter.
Where:
$d_w$ = Wire diameter (m)
$\varepsilon$ = Wick porosity
$N$ = Mesh number (lines per meter)
This shows how the fine geometry you choose for the wick directly scales the maximum heat the pipe can handle.
Frequently Asked Questions
The operating temperature may be too close to the boiling or freezing point of the working fluid, or the effective capillary radius of the wick may be too large. Please check the temperature range and change the wick type to recalculate.
After selecting each fluid, move the operating temperature slider, and temperature-dependent properties such as surface tension and latent heat will be plotted on the graph in real time. You can overlay curves of different fluids for comparison.
Effective thermal conductivity is an indicator for estimating the overall thermal resistance of the heat pipe. A higher value means better heat spreading performance. In thermal design, use this value to determine the required number and arrangement of heat pipes.
Increasing the pipe length raises pressure loss, reducing the capillary limit heat transport capacity. Conversely, increasing the cross-sectional area decreases the heat flux. Adjust the dimensions according to the design objectives.
Real-World Applications
Spacecraft Thermal Control: Heat pipes are vital in satellites, where they move heat from electronics to radiators without moving parts. Ammonia heat pipes are common for their wide temperature range and high performance in zero-gravity, where capillary action is the only reliable pumping mechanism. Engineers use calculations like this to size pipes for instrument panels.
High-Performance Electronics Cooling: From gaming PCs to data center servers, heat pipes efficiently transport heat from a hot CPU to a large finned heatsink. Copper-water heat pipes with sintered powder wicks are typical here, chosen for their high capillary limit at near-room temperature, which you can test in the simulator.
Energy Recovery & HVAC: Heat pipe heat exchangers pre-cool incoming fresh air in buildings by transferring heat to the exhaust air stream. Arrays of moderate-temperature heat pipes (using methanol or acetone) are used. Their design relies on knowing the capillary limit to ensure each pipe operates reliably over decades.
Nuclear Reactor Cooling: In some advanced reactor designs, heat pipes are proposed as a passive, fail-safe cooling system for the core. Liquid metal heat pipes (e.g., sodium or potassium) operating at very high temperatures would be used, where accurately predicting the capillary limit is a critical safety calculation.
Common Misconceptions and Points to Note
When you start using this tool, there are a few common pitfalls to watch out for. First, understand that "the calculation results are ideal values". The capillary limit provided by the tool is a theoretical maximum. Actual performance can be 10-30% lower due to wick uniformity, impurities, or tilt (gravity effects). For instance, even if the calculation shows 100W, the practical wisdom is to set your design heat load to 70W for a safety margin.
Next, pay attention to the "Temperature" parameter setting. You should input the intended 'operating' average temperature after the heat pipe has started functioning as designed, not the temperature of the heat source itself. For example, if you're designing a CPU cooler to operate at 80°C, evaluate the working fluid properties around 80°C. Calculating with properties at 0°C is meaningless.
Finally, be mindful of the interpretation of "Effective Thermal Conductivity". The value shown by the tool is an apparent value assuming the entire heat pipe is a "homogeneous solid". So, don't get overly excited about "1000 times that of copper!" and try to use that value in conventional thermal conduction calculations. Use it strictly as a guideline for incorporating the heat pipe as a "component with minimal thermal resistance" into your system.
Select water, ammonia, acetone, or methanol as the working fluid; the simulator updates surface tension, latent heat, liquid viscosity, and density automatically.
Enter operating temperature, outside diameter, and effective length within the slider ranges.
Select sintered, grooved, or mesh wick to apply the corresponding permeability and wick area assumptions.
The page updates capillary limit, effective conductivity assuming a 5 K axial temperature difference, maximum heat flux, and latent heat in real time.
Worked Example
Copper-water heat pipe, 6 mm OD, 0.20 m effective length, water at 60°C, sintered wick. With the patched property model, latent heat is about 2359 kJ/kg and surface tension about 66.1 mN/m. The calculator gives capillary limit ≈ 452 W, k_eff ≈ 640 kW/(m·K), and maximum heat flux ≈ 1599 W/cm².
Practical Notes
The chart shows the capillary limit Q[W] versus temperature for the four supported fluids; ammonia is truncated above its critical-temperature guard.
Water performs well above room temperature, while ammonia is mainly useful in low-temperature ranges where water would freeze.
In vertical installation, hydrostatic head ρgL can reduce the available capillary pressure, especially for water.
Do not use this apparent effective conductivity as a bulk material constant; use it as a component-level thermal-resistance estimate.