Why are the transient temperature charts prepared using nondimensionalized quantities such as the Biot and Fourier numbers instead of the actual variables such as thermal conductivity and time?

First, let's define the Biot number (Bi) and Fourier number (Fo). The Biot number is a dimensionless parameter that characterizes the ratio of internal thermal resistance to external (convective) thermal resistance, and mathematically it is defined as: \[Bi = \frac{hL}{k}\] where \(h\) is the heat transfer coefficient (W/m²K), \(L\) is the characteristic length (m), and \(k\) is the thermal conductivity (W/mK). The Fourier number, on the other hand, is a dimensionless parameter relating to the rate of heat conduction and the rate of heat storage, and is given by: \[Fo = \frac{\alpha t}{L^2}\] where \(\alpha\) is the thermal diffusivity (m²/s) and \(t\) is the time (s).

Nondimensionalization is the process of scaling quantities to remove dimensions, and it's useful in several ways: 1. Simplifying complex equations: By using dimensionless quantities, we can simplify complex equations that involve multiple parameters and dimensions into simpler forms. 2. Easier comparison of different systems: With dimensionless quantities, it's easier to compare different systems that may have widely varying scales or units. This makes it easy to generalize the results for a wider range of applications. 3. Reduced number of variables in an equation: Using nondimensionalized quantities decreases the number of variables in an equation, making it easier to solve and analyze. 4. Universality: Nondimensionalization allows the equation to be valid across different systems, regardless of the initial units being used.

Using Biot and Fourier numbers in transient temperature charts offers several advantages: 1. Easier comparison: By using Biot and Fourier numbers, we eliminate units associated with thermal conductivity, heat transfer coefficient, characteristic length, and time, enabling easier comparison of transient temperature charts for various conditions and systems. 2. Generalization: Using nondimensionalized quantities like Biot and Fourier numbers allows us to develop a single chart for various conditions and systems that can be easily referenced and scaled to actual variables as needed. 3. Easier interpretation: Transient temperature charts with nondimensionalized quantities are often easier to interpret, as they provide a simple method to look at trends and patterns across various systems and conditions, without involving the complexities of actual variables. In conclusion, preparing transient temperature charts using Biot and Fourier numbers, instead of actual variables like thermal conductivity and time, provide us with a more generalized and easily interpreted chart. The nondimensionalized quantities make it possible to draw comparisons across various systems and conditions, simplifying the analysis and understanding of temperature-dependent processes.