Laminar flow does not transfer energy as well as turbulent flow. The reason for the poorer heat transfer is the laminar flow that is always obtained near a plane wall. The heat transfer rate within the film is significantly lower than in the bulk liquid, because the temperature gradient decreases dramatically in this area (see Figure 1.5). The space between the dotted lines and the wall in Figure 1.5 is often called the film thickness. In brazed plate heat exchangers, the energy is therefore transferred through conduction and convection, and examples of these types of energy transport are shown in In BPHEs, the energy is therefore transferred through conduction and convection, and examples of these types of energy transport are shown in Figure 1.4. Radiation does not contribute significantly to the heat transfer in brazed plate heat exchangers, due to their considerably lower working temperature.
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