Heat and Mass Transfer for Biological Systems
Pickles
Learning about heat and mass transfer has changed the way I look at something as simple as pickles. Before, I honestly just though of pickling as soaking cucumbers in vinegar, but now I understand that it's actually a complex combination of heat transfer, mass diffusion, and equilibrium processes happening at the same time.
Something that stands out to me is how mass transfer governs the pickling process. When cucumbers are places in a brine solution, salt and acetic acid diffuse into the cucumber while water diffuses out. This is driven by concentration gradients, over time moving the system to equilibrium. This process isn't simple, it depends on factors like temperature, concentration, and structure of the cucumber. Something as small as slicing the cucumber thinner can significantly speed up diffusion because it reduces the distance mass has to travel.
Heat transfer also plays a critical role. The basic convection equation can show how heat moves from the hot brine into the cucumber surface.
Then Fourier's Law, or conduction heat transfer, comes in, which helps determine how heat travels through the solid cucumber from the outside to the inside.
Concepts like the Biot number help determine whether the temperature within the pickle can be assumed uniform or if there are significant internal gradients.
Solar Panels
My parents recently had solar panels installed on our house, which made me think about how they work beyond "using sunlight." Now I understand that their performance is tied to heat transfer, especially radiation and how energy is exchanged between the sun and panel surface. The Stefan-Boltzmann equation helps make sense of the radiation heat transfer that solar panels use.
Something I find interesting about this is that solar panels are not just absorbing light, they are constantly emitting energy back into their surroundings. This means that their efficiency depends on balancing how much radiation is absorbed vs how much is lost. It also shows how much material properties matter. Panels are designed with high absorptivity to capture as much incoming solar radiation as possible, while also managing emissivity. Convection also plays a role since wind or air flow can cool the panels, which surprisingly helps because most solar panels perform better at lower temperatures.
Water Pipes
One application we talked about in class that piqued my interest is why pipes freeze in some climates, and not others, and why people in warmer regions have to drip their faucets during cold spouts. The main idea is that freezing depends on how quickly heat is lost from inside the pipe to the surrounding environment. This is largely governed by convection from the pipe surface to the air.
In warmer climates, when the air temperature suddenly drops, the temperature difference between the pipe surface and the surrounding air becomes large, causing the water inside the pipe to cool much more quickly. Heat must also conduct through the pipe wall before it can be lost to the environment, which is described by Fourier's law.
In colder climates, pipes are often buried deeper underground or have better insulation to reduce heat transfer. This means that the surrounding soil helps maintain a more stable tempterature and slows the cooling process down significantly.
To understand how water temperature changes over time, transient heat transfer can be used.
This equation shows that the temperature of the water decreases exponentially over time depdning on convection, material properties, and system geometry. Dripping faucets works as a practical solution because it introduces flow into the system. Moving water continuously brings in slightly warmer water from the plumbing system and prevents stagnation, which delays the system from reaching freezing conditions.
