# Conjugate Heat Transfer Analysis and Heat Dissipation Design of Nucleic Acid Detector Instrument
## 4 Simulation Results and Discussion
### Heat-Dissipation Scheme  
During operation, the fan circulated the air inside the instrument, and over time the ambient internal temperature increased. High ambient temperatures threaten the stability of the electronic components. The analysis in Section 4.3 showed that the heat was concentrated at the top-back section of the instrument; based on this, two heat-dissipation schemes were designed, as shown in Figure 21. In Plan 1, an inlet was positioned at the bottom of the front of the instrument and an outlet was positioned at the top of the back (Figure 21a). In Plan 2, inlets were positioned around the bottom sides of the instrument and an outlet was positioned at the top (Figure 21b). In both cases, the area of the inlet was approximately equal to the area of the outlet.  
There are fans for the amplification modules inside the instrument. According to the pressure difference obtained by simulation, combined with the pressure polynomial in Section 3.2, it is estimated that the velocity is between 5.5-6 m\(\cdot\)s\({}^{-1}\) (flow rate is between 0.014-0.016 m\({}^{3}\cdot\)s\({}^{-1}\)). Considering that the volume of the fluid domain inside the entire instrument was 0.038 m\({}^{3}\), a silent fan with a maximum flow rate of 0.028 m\({}^{3}\cdot\)s\({}^{-1}\) was preliminarily selected (Delta Electronics, AFB0924 HH). Thus, the pressure polynomial was:  
\[\Delta P=53.935-12.953v-0.3001v^{2} \tag{15}\]  
The temperatures at p1-p9 at the last moment of the simulation were used as the evaluation indexes. Figure 22 shows the temperatures at each point under each scheme. At each point, the temperature under Plan 2 was lower than that under Plan 1; therefore, Plan 2 had a superior heat-dissipation effect. Figure 23 shows the temperature and flow velocity of the section and the overall streamline of the Plan-2 model at the last moment of the simulation. The fluid entered the instrument at a high velocity and then slowed as it became blocked by the internal structure of the device. The flow velocity was increased to remove heat under the action of the fan on the amplification module.  
Figure 21: Heat-dissipation schemes: (**a**) plan 1; (**b**) plan 2.  
Figure 22: Temperature at points p1–p9 under the different schemes.  
The trends in the simulated and experimental temperatures at points p1-p9 under Plan 2 are shown in Figure 24. The relative error of each point did not exceed \(\pm\)2 K and the maximum relative error of p9 was 0.54%. The acrylic casing of the prototype based on Plan 2 was used to evaluate the heat-dissipation capability. Figure 25a shows the prototype of the casing and instrument body. Nucleic acid amplification was conducted continuously for 120 min and the temperatures of p1-p9 were measured simultaneously, as shown in Figure 25b. The maximum temperature was 307.76 K. During the first amplification process, the temperature increased rapidly and then stabilized in subsequent amplification cycles. After each amplification, the temperature decreased significantly, which indicates that the residual heat was removed from the instrument immediately after each amplification. This shows that the proposed heat-dissipation scheme for nucleic acid detectors is feasible.  
Figure 23: (**a**) Section temperature at x = 0.038 m; (**b**) section velocity at x = 0.038 m; (**c**) overall streamline.  
Figure 24: Variation trends in simulation value and experimental value of points p1–p9.