Secure & Private (Zero Data Retention)
Free Access • No Sign-Up
💧 Working Fluid & Saturation Temp
°C
Watts (W)
degrees (°)
📏 Geometry & Wick Specifications
mm
mm
mm
porosity
⚙️ Section Lengths & Thermal Network
mm
mm
mm
📊 Heat Pipe Operational Limits & Thermal Network
Capillary Limit (Q_cap):264 W
Sonic Choking Limit (Q_sonic):1,420 W
Entrainment/Flooding Limit:412 W
Boiling Burnout Limit (Q_boil):380 W
Absolute Heat Transport Limit:264 W (Capillary)
Thermal Safety Margin:1.47x (SAFE)
Figure of Merit (M):9.42 × 10⁹ W/m²
Capillary Pumping Head:5.12 kPa
Vapor Core Diameter:9.0 mm
Total Thermal Resistance (R_th):0.062 °C/W
End-to-End Temp Drop (ΔT):11.2 °C
Equivalent Effective Cond:28,500 W/m·K
Heat Pipe Internal Two-Phase Cycle & Transport Limits vs TemperatureCapillary, Sonic, Entrainment & Boiling Envelopes
Fatal Traps & Industrial Operating Hazards
1. Non-Condensable Gas (NCG) Poisoning & Condenser Choking
Incompatible container/fluid pairings (e.g. water inside aluminum or carbon steel) generate hydrogen gas through chemical oxidation: 2Al + 3H2O -> Al2O3 + 3H2. Because hydrogen is non-condensable, vapor flow sweeps it into the cold condenser tip, creating a stagnant gas plug. This blankets heat transfer surfaces, choking effective condenser length by 50% to 90% and causing the evaporator temperature to skyrocket within weeks of commissioning.
2. Gravity Tilt Inversion in Sintered vs Grooved Heat Pipes
Operating a heat pipe with the evaporator located above the condenser against gravity (adverse tilt, phi < 0°) forces the capillary wick to pump condensate against hydrostatic head: Delta P_g = rho_l * g * L * sin(phi). Axially grooved wicks (r_eff > 100 µm) generate less than 1.2 kPa of capillary suction; tilting them even 5 degrees against gravity completely halts liquid return and induces instant dryout. Sintered powder wicks (r_eff < 25 µm) are strictly mandatory for zero-g or against-gravity orientation.
3. Liquid Dryout from Vapor Core Shear Entrainment
At high heat flux throughputs, the counter-flowing vapor core velocity can exceed 30 m/s. Interfacial shear strips liquid droplets from the porous wick surface (Kutateladze entrainment limit). If the vapor core diameter is pinched down (e.g. excessive wick thickness in a small 6 mm pipe), the vapor Mach number spikes, entraining condensate before it reaches the evaporator and causing premature dryout far below the theoretical capillary limit.
4. Over-Filling vs Under-Filling Charge Catastrophes
The working fluid charge volume must precisely match the open pore volume of the wick plus a microscopic meniscus reservoir (typically 105% to 115% of wick void volume). Under-charging leaves wick dry spots at the evaporator hot spot, causing localized burning. Over-charging creates a free liquid puddle that accumulates at the condenser end, insulating heat transfer surfaces and acting as a virtual NCG slug that degrades overall thermal conductance.
5. Freeze-Thaw Envelope Rupture during Cold Storage
Water-charged heat pipes deployed in aerospace, telecom, or sub-zero ambient conditions expand by 9% upon freezing. If pure water freezes in a solid liquid plug within the vapor core without void clearance, subsequent rapid evaporator startup creates high local hydraulic pressures trapped against frozen condenser ice. The copper envelope bulges, delaminates the sintered wick from the container wall, and eventually bursts the tube seams.
What is the working fluid Figure of Merit (M) and why does it govern heat pipe performance?▼
The Figure of Merit M = (rho_l * sigma * h_fg) / mu_l combines liquid density (rho_l), surface tension (sigma), latent heat of vaporization (h_fg), and dynamic liquid viscosity (mu_l). It directly represents the liquid phase’s intrinsic ability to transport latent heat through a capillary wick against viscous drag. Water has the highest Merit number of all common moderate-temperature fluids (~10^10 W/m² at 100°C), vastly superior to methanol or acetone, making it the ideal working fluid between 30°C and 250°C.
How does the capillary limit restrict maximum heat transport capacity?▼
A heat pipe operates passively by relying on capillary pressure generated within the wick pore menisci at the evaporator: Delta P_cap = 2 * sigma / r_eff. For continuous operation, this capillary head must overcome the entire closed-loop pressure drop: Delta P_cap >= Delta P_liquid + Delta P_vapor + Delta P_gravity. If the thermal heat throughput exceeds Q_cap, viscous friction in the liquid wick exceeds the capillary pumping head, causing the liquid menisci to recede into the wick until the evaporator dries out (dryout crisis).
What causes the entrainment (flooding) limit in thermosyphons and heat pipes?▼
In a heat pipe or vertical thermosyphon, high-velocity vapor travels from evaporator to condenser while liquid condensate flows counter-currently back along the wick or tube wall. When vapor velocity exceeds the critical Kelvin-Helmholtz instability threshold (governed by the Kutateladze number K = 3.2), interfacial shear waves rip liquid droplets off the wick into the high-speed vapor core. This entrainment starves the evaporator of condensate, triggering instantaneous wall temperature runaway.
What is the sonic limit and when does it throttle heat pipes during startup?▼
During frozen startup or operation near the triple point, vapor density is extremely low. To transport even moderate heat fluxes, the vapor must accelerate to high velocities. If vapor speed at the exit of the evaporator section reaches the local speed of sound (Mach 1), the vapor flow chokes. Beyond this point, lowering condenser pressure or temperature cannot increase vapor mass flow rate, establishing an absolute thermodynamic ceiling known as the sonic limit.
Why do sintered copper-powder wicks outperform axial grooves in high heat flux electronics?▼
Sintered metal powder wicks provide extremely small effective pore radii (r_eff = 10 to 40 µm), generating powerful capillary heads exceeding 5 to 15 kPa. Furthermore, the porous sintered copper matrix has a high effective thermal conductivity (k_eff = 20 to 40 W/(m·K)), delaying the onset of nucleate boiling burnout. In contrast, grooved wicks have large hydraulic diameters (r_eff = 100 to 250 µm) with low capillary heads that fail against gravity tilt.
Frequently Asked Questions
What is the working fluid Figure of Merit (M) and why does it govern heat pipe performance?+
The Figure of Merit M = (rho_l * sigma * h_fg) / mu_l combines liquid density (rho_l), surface tension (sigma), latent heat of vaporization (h_fg), and dynamic liquid viscosity (mu_l). It directly represents the liquid phase’s intrinsic ability to transport latent heat through a capillary wick against viscous drag. Water has the highest Merit number of all common moderate-temperature fluids (~10^10 W/m² at 100°C), vastly superior to methanol or acetone, making it the ideal working fluid between 30°C and 250°C.
How does the capillary limit restrict maximum heat transport capacity?+
A heat pipe operates passively by relying on capillary pressure generated within the wick pore menisci at the evaporator: Delta P_cap = 2 * sigma / r_eff. For continuous operation, this capillary head must overcome the entire closed-loop pressure drop: Delta P_cap >= Delta P_liquid + Delta P_vapor + Delta P_gravity. If the thermal heat throughput exceeds Q_cap, viscous friction in the liquid wick exceeds the capillary pumping head, causing the liquid menisci to recede into the wick until the evaporator dries out (dryout crisis).
What causes the entrainment (flooding) limit in thermosyphons and heat pipes?+
In a heat pipe or vertical thermosyphon, high-velocity vapor travels from evaporator to condenser while liquid condensate flows counter-currently back along the wick or tube wall. When vapor velocity exceeds the critical Kelvin-Helmholtz instability threshold (governed by the Kutateladze number K = 3.2), interfacial shear waves rip liquid droplets off the wick into the high-speed vapor core. This entrainment starves the evaporator of condensate, triggering instantaneous wall temperature runaway.
What is the sonic limit and when does it throttle heat pipes during startup?+
During frozen startup or operation near the triple point, vapor density is extremely low. To transport even moderate heat fluxes, the vapor must accelerate to high velocities. If vapor speed at the exit of the evaporator section reaches the local speed of sound (Mach 1), the vapor flow chokes. Beyond this point, lowering condenser pressure or temperature cannot increase vapor mass flow rate, establishing an absolute thermodynamic ceiling known as the sonic limit.
Why do sintered copper-powder wicks outperform axial grooves in high heat flux electronics?+
Sintered metal powder wicks provide extremely small effective pore radii (r_eff = 10 to 40 µm), generating powerful capillary heads exceeding 5 to 15 kPa. Furthermore, the porous sintered copper matrix has a high effective thermal conductivity (k_eff = 20 to 40 W/(m·K)), delaying the onset of nucleate boiling burnout. In contrast, grooved wicks have large hydraulic diameters (r_eff = 100 to 250 µm) with low capillary heads that fail against gravity tilt.