# Numerical Simulation and Design of a High-Temperature, High-Pressure Fluid Transport Pipe
## 1 Introduction  
When designing any product, reliability and performance should be at the forefront. However, even with the best designs, unexpected problems can occur during use. For example, in the case of a laptop or a smart device, reliability and performance are guaranteed. However, low-temperature burns occur frequently during their use. Thus, it is necessary to design products that emphasize and prioritize safe use. For this reason, this study addresses the design of a hand-caliber that uses a high-temperature, high-pressure internal fluid transport pipe (i.e., barrel), in consideration of reliability, safe use, and performance. In terms of reliability, the strength of the hand caliber is checked to prevent damage from high-temperature, high-pressure internal gas. In terms of safe use, low-temperature burns caused by contact and high-temperature burns caused by high-temperature gas leakage in the cylinder gap is considered. In terms of performance, the projectile velocity is considered. Considering all theseaspects, the experimental approach for designing a hand-caliber becomes quite time consuming and expensive, as compared to the mathematical approach or the numerical method. Moreover, numerical methods are mainly used as tools for predicting the temperature distribution, stress, and other fluid mechanisms. When approaching a hand-caliber design that accounts for reliability, safe use, and performance by numerical methods, heat-transfer, structural, and gas-leak analyses are required. The related studies are as follows:  
(1) Heat transfer is primarily caused by the force convection from high-temperature combustion gases generated inside the barrel during firing. As barrel wear is an important consideration in design and is very closely related to the inner surface temperature, many studies on barrel-heat transfer were conducted [1]. Most researchers are interested in determining total heat transfer and barrel-temperature distribution. Barrel-heat transfer is obtained using thermo-chemical approaches, lumped-parameter methods, and inverse-heat conduction modeling. Barrel-temperature distributions were predicted by numerical simulation and were experimentally verified [2; 3; 4; 5; 6]. (2) The stress analysis of the hand caliber was caused by thermo-mechanical loading at high-temperatures and pressures inside the barrel. Many researchers analyzed stress, as it is related to hand caliber failure. Energy-based methods are used to obtain a simple analytical solution for the plastic stress of a thick-walled steel cylinder and to analyze that of the barrel affected by dynamic pressure [7]. Using a numerical simulation of the dynamic loading process of the barrel, researchers examined the radial effects of gas pressures [8]. The transient heat flux inside the barrel was obtained by using the conjugate gradient method; thus, temperature distributions and thermal stresses were predicted [9]. Using the heat-transfer and structure-coupled finite element analysis method, the strength of the barrel under thermo-mechanical loads was evaluated, and a study was performed on the thermo-mechanical stress response through projectile friction, contact pressure between projectile and barrel, and grain size and initial temperature of double-base propellants [10; 11; 12]. (3) Gas-leakage analysis requires computational fluid dynamics (CFD) simulation. The main CFD simulation studies of muzzle flow deal with the unsteady muzzle-blast phenomenon that affects the firing accuracy [13; 14]. The transient convective heat-transfer coefficient of the barrel is required for heat transfer and structure analysis of the hand caliber. However, few studies were conducted in this regard. Although thermal stress and mechanical stress caused by thermo-mechanical loading were approached independently, there were none that compared thermal and mechanical stresses. Additionally, it was necessary to predict the surface temperature of the hand caliber to evaluate low-temperature burns.