The NASA Lewis Research Center Heat Pipe (LERCHP) code was developed to predict the performance of heat pipes in the steady state. LERCHP can be used as a design tool on a personal computer or, with a suitable calling routine, as a subroutine for a mainframe radiator code. For accurate heat pipe modeling, a variety of wick structures are available to the user, including a user wick input option. Several working fluids can be chosen as well, including potassium, sodium, and lithium, for which the monomer-dimer equilibrium is considered. A vapor flow algorithm is incorporated that treats compressibility and axially varying heat input. LERCHP also facilitates the determination of heat pipe operating temperatures and heat pipe limits that may be encountered at the specified heat input and environment temperature. LERCHP has been written for straight, cylindrical heat pipes with no bends. Simple modification to noncircular cross sections could be achieved by using the hydraulic radius approach, but this feature is not currently implemented. Curvature without sharp bends is permissible. Operation in the presence of gravitational fields is accounted for in the case of straight pipes, but thermosyphons are not included.

Heat pipes with multiple evaporators, condensers, and adiabatic sections in series and with wick structures that differ among sections can be modeled by LERCHP. The heat pipe can be subdivided into as many as 20 sections, far more than will normally be required. These sections can be heat input or evaporator sections, adiabatic sections, and heat removal or condenser sections, mixed in any manner provided that the first section is a heat input section.

The nature of any section is indicated by the type of boundary condition data that is entered for that section. At present the following options are available for the boundary conditions:

1. specified heat input or removal rate at the pipe exterior surface and
   
   
2. specified environment temperature external to the pipe, with heat transfer conditions from the pipe surface to the environment provided by the user.

Output, such as liquid and vapor pressures and temperatures, is printed at equally spaced axial positions along the pipe as determined by the user. LERCHP allows the user to select the number of uniformly spaced points along the pipe axis at which the principal variables will be printed out. The spacing of these points need not correspond with the step size chosen for the Runge-Kutta solution. A printout is furnished after thermal convergence of the solution has been obtained. When a heat pipe limit is encountered during calculation of the solution, but does not cause the calculation to halt, the type of limit is printed out with the solution. Output can be directed to a plotting device if a suitable plotting utility is available and the necessary changes are made in the source code.