The study considered development of a finite-element numerical simulation model for the analysis of fluid flow and conjugate heat transfer in a zero boil-off (ZBO) cryogenic storage system. A spherical tank was considered for the investigation. The tank wall is made of aluminum and a multi-layered blanket of cryogenic insulation (MLI) has been attached on the top of the aluminum. The tank is connected to a cryocooler to dissipate the heat leak through the insulation and tank wall into the fluid within the tank. The cryocooler has not been modeled; only the flow in and out of the tank to the cryocooler system has been included. The primary emphasis of this research has been the fluid circulation within the tank for different fluid distribution scenario and for different level of gravity to simulate all the potential earth and space based applications. The steady-state velocity, temperature, and pressure distributions were calculated for different inlet positions, inlet velocities, and for different gravity values. The simulations were carried out for constant heat flux and constant wall temperature cases. It was observed that a good flow circulation could be obtained when the cold entering fluid was made to flow in radial direction and the inlet opening was placed close to the tank wall. The transient and steady state heat transfer for laminar flow inside a circular microtube within a rectangular substrate during start up of power has also been investigated. Silicon, Silicon Carbide and Stainless Steel were the substrates used and Water and FC-72 were the coolants employed. Equations governing the conservation of mass, momentum, and energy were solved in the fluid region. Within the solid wafer, the heat conduction was solved. The Reynolds number, Prandtl number, thermal conductivity ratio, and diameter ranges were: 1000--1900, 6.78--12.68, 27--2658, and 300 µ m--1000 µ m respectively. It was found that a higher aspect ratio or larger diameter tube and higher thermal conductivity ratio combination of substrate and coolant requires lesser amount of time to attain steady state. It was seen that enlarging the tube from 300 µ m to 1000 µ m results in lowering of the fluid mean temperature at the exit. Nusselt number decreased with time and finally reached the steady state condition. It was also found that a higher Prandtl number fluid attains higher maximum substrate temperature and Nusselt number. A correlation for peripheral average Nusselt number was developed by curve-fitting the computed results with an average error of 6.5%. This correlation will be very useful for the design of circular microtube heat exchangers.