1 heat dissipation method of power LED lamps
The heat dissipation methods of high-power LED lamps mainly include active heat dissipation and passive heat dissipation, which are summarized below.
(1) Active heat dissipation, including the addition of fan forced cooling, water cooling technology, and semiconductor cooling chip. In the case of streetlight applications, there is a problem with the stability of the outdoor installation fan. Water-cooled heat dissipation and semiconductor refrigeration chips require extra power, and the heat dissipation end requires an additional heat dissipation structure. This is generally avoided in LED lighting applications. design.
(2) Passive heat dissipation, including natural heat dissipation, heat pipe fins, temperature plate and fins, and loop heat pipe technology.
Natural heat dissipation uses heat conduction, heat convection, and heat radiation as the basic heat dissipation principle. The main purpose is to use heat conduction to conduct heat to the outer wall of the luminaire, and then dissipate heat by convection of the surface area of ​​the luminaire or the convection of the fins and the surrounding air and radiate heat to the surrounding objects. This is the most common way of dissipating heat.
The heat pipe is finned, and the collected heat is transferred to the heat pipe terminal through the heat pipe end, and then the fin heat sink is installed at the heat pipe terminal to dissipate the heat. At present, the method is relatively mature and is most widely used in high-end computers.
The uniform temperature plate and fins, the principle of the uniform temperature plate and the heat pipe are the same as the theoretical structure. Only the direction of heat conduction is different. The heat conduction mode of the heat pipe is one-dimensional, which is a linear heat conduction mode, and the heat conduction mode of the uniform temperature plate is two-dimensional. It is the way of heat conduction.
The heat pipe of the circuit dissipates heat. When heat is transferred from the evaporator to the working medium in the loop heat pipe, the working medium absorbs heat and evaporates to the condenser, releases the heat and condenses, and then returns to the evaporator by the capillary force of the porous material in the evaporator. , repeat the loop. In the original passive heat sink, the loop heat pipe just fills the shortcoming that the heat pipe and the temperature equalizing plate cannot transfer heat from a long distance, and can transfer the heat to a place far away or easier to dissipate heat. The loop heat pipe technology was originally applied to space technology, artificial satellites, etc. In LED lighting is a new attempt, the principle is as follows (a), the application is as shown in (b), the heat pipe can use the lamp housing to dissipate heat, no need to install more fins. If the aesthetics are considered, the heat pipe can be placed inside the lamp housing.
Due to the wide variety of heat dissipation structures, it is necessary to select according to the respective characteristics of the products in the specific application process of the heat dissipation structure selection of LED lamps, and to optimize the structure. The optimized design of the LED heat dissipation structure can be generally divided into three levels: device level, board level, and system level.
The first two focus on the internal structural optimization of electronic devices, including device-level optimization of heat sinks and board-level device layout optimization. Compared with the device-level and board-level thermal design, the thermal design of the LED luminaire system level has a more significant impact on the thermal performance. The frame form of the heat sink has a decisive influence on the flow field, which plays a crucial role in the heat dissipation performance. A reasonable system-level thermal design can greatly improve the temperature distribution of LED lamps and reduce the maximum temperature.
In the LED heat dissipation design, the finned heat sink has been widely used due to its simple structure, convenient processing and good heat dissipation effect. It consists of ribs and pedestal. The main geometric parameters include rib length, rib thickness, number of ribs, base thickness, base width and so on. Considering that the LED lamps studied in this paper are used in the civil field, the fin-type radiators are selected as the research object, and the thermal performance of the power LED lamps is numerically analyzed and optimized.
2 LED heat dissipation structure analysis model
2. 1 basic equation of heat conduction
In the general three-dimensional problem, the steady-state heat conduction equation is xkx + yky + zkz + Q = 0(1)kx, ky, kz are the thermal conductivity of the thermal conductor along the x, y, z direction, respectively, which is the thermal mass density, Q is The heat source density inside the object (in W/kg).
Boundary conditions: the first type of boundary conditions: (on the 1 boundary) = (2) the second type of boundary conditions: (on the 2 boundary) kxnx + kyny + kznz = q (3) the third type of boundary conditions: (at 3 On the boundary) kxnx + kyny + kznz = h(a - )(4)nx, ny, nz is the direction cosine of the normal outside the boundary; = ( , t) is the given temperature at the boundary of 1; q = q( , t) is the given heat flux density (W/ m 2) at the 2 boundary; h is the convective heat transfer coefficient (W/ m 2 K); a = a( , t) , for the 3 boundary, in the natural convection condition Next, a is the ambient temperature; under forced convection, a is the adiabatic wall temperature of the boundary layer.
2. Establishment of a 2-bar radiator model
The exact solution of the above heat conduction equation is usually difficult, especially considering the coupling of the flow field and the temperature field in the natural convection process, the solution of the equation becomes more difficult. In this study, the company's ANSYS company's professional electronic product thermal analysis software ICEPAK for numerical analysis and parameter optimization design of LED heat dissipation structure. The structure of the strip LED luminaire is shown as 2.
In the process of establishing the model, the overall structure of the luminaire is divided into three parts, which are respectively referred to as A, B and C. A is an LED heat source, B is an aluminum substrate and a thermal silicon gasket (the aluminum substrate is composed of aluminum, an insulating layer, and copper), and the LED heat source is connected to the aluminum substrate through a package, and C is an aluminum alloy heat sink frame. The overall size of the model X YZ is 34. 8mm 24. 4 mm 540 mm.
In ICEPAK, a rectangular cabinet is used to simulate the entire calculation area, as shown by the blue line. The boundary condition of each side adopts the opening model to simulate the natural convection condition, and the isothermal boundary condition is applied to the boundary of the Cabinet, which is room temperature 25.
The aluminum substrate consists of three layers of material, with an aluminum thickness of 2 mm, a thermal conductivity of 150 W/mK, an insulating layer thickness of 35 m, a thermal conductivity of 1.3 W/mK, a copper layer thickness of 35 m, and a thermal conductivity of 400 W/ mK. For the insulating layer and the copper layer, due to the extremely small thickness, the plate model is used to model it in ICEPAK. In the calculation, the thickness direction is not meshed, and the specific model is shown in 4.
In the LED luminaire, there are a total of 20 LED heat sources arranged in the Z direction. As shown in the figure, a single LED heat source structure is as shown in (a), each heat source has a power of 1 W, a heating power of 0.8 W, and a package thermal resistance of 15 / W. Due to the complex internal structure of the LED lamp package heat source, it is difficult to accurately simulate. In the specific modeling, a simplified method is used to simulate the packaged LED heat source with a heat source and a thermal resistance chip. The specific model is shown in (b). The yellow area is a heat resistant sheet and the red area is a heat source.
3 numerical examples and discussion
3. Analysis of heat dissipation structure of 1 strip radiator
The above calculation model was numerically simulated in the ICEPAK software to obtain the temperature field and flow field distribution results of the radiator structure and the luminaire. In order to more clearly reflect the temperature distribution of the luminaire and the distribution of the surrounding flow field, we respectively give the section temperature diagram and the section flow field diagram, and the temperature distribution diagram of the aluminum alloy heat dissipation frame. Considering that the strip LED illuminator is working, the middle temperature is higher than the two ends. Figure 6 shows the temperature field and flow field distribution of the cross section in the heat sink. In order to reflect the temperature of the LED heat source, the cross-sectional temperature distribution of the central part of the LED lamp through the heat source is given as shown.
The temperature field distribution diagram of the section in the radiator (Z= 270 mm, the section does not pass the LED heat source) The flow field distribution of the middle section The different color in the section temperature distribution (Z= 270 mm) of the heat source indicates the temperature field height and the length of the arrow. The direction and direction respectively indicate the magnitude and flow direction of the air flow rate.
It can be seen from the above that the maximum temperature of the strip aluminum alloy heat dissipation frame is about 59, and the maximum temperature of the LED heat source is about 71.
The technicians of Liangshuo Company of Shanghai Academy of Sciences used the thermocouple to measure the maximum temperature of the aluminum alloy heat dissipation frame. The test result is 60. This verifies the correctness of the numerical model and analysis method used in this paper to some extent.
The overall temperature field distribution of the aluminum alloy heat dissipation frame is as shown.
Observing the temperature field distribution of the aluminum alloy heat dissipation frame, it can be seen that the high temperature fluid (middle red region) on the left side of the LED heat source is concentrated, but due to the closed structure, the flow velocity vector is almost zero, which causes the accumulated heat to be effectively lost through natural convection. Go to the surrounding air. At the same time, the area with faster flow rate (the longer middle arrow area) is far away from the heat source, and the fluid temperature is relatively low, mainly distributed at the outermost boundary, which makes the heat taken away by the fluid less. The above two factors are the temperature of the LED lamp. The main reason for the higher.
3. Improvement and optimization design of 2 strip radiator structure Based on the analysis of the previous section, in order to improve the fluidity of the high temperature region on the left side of the LED heat source, we open 11 rectangular holes on both sides of A and B in 0, the size of the hole is 6. 65. Mm 28 mm, the hole-to-hole spacing is 19 mm. The specific model is shown as 0.
The improved strip radiator structure model uses ICEPAK software to recalculate the improved model. The cross-section temperature field distribution in the improved heat dissipation structure and the flow field distribution through the middle section are shown in Figures 1 and 2. The temperature field distribution through the heat source and the overall temperature distribution of the aluminum alloy heat dissipation frame are shown in Figures 3 and 4.
The improved heat dissipation structure is improved by the improved temperature distribution of the aluminum alloy heat dissipation frame in the left region 14 of the LED heat source, and the flow in the left region near the heat source is smoother and the flow field distribution is more reasonable. It can be seen from 2 that the closed high temperature region which is close to the heat source has the highest flow velocity of the fluid flow, and the fluid temperature is also high. Such a flow field distribution can efficiently carry away the heat sink accumulated near the heat source. The heat. It can be clearly seen from Fig. 11 and Fig. 13 that the maximum temperature of the LED heat source and the aluminum alloy frame is reduced by about 7 compared with the original design, and the weight of the optimized heat dissipation frame is 4. 45% lower than that of the prototype heat dissipation frame.
3. 3 hole size and number of optimized design
3. 3. 1 The optimal design of the hole size is to keep the number of holes as 11, and other geometric factors are unchanged. Change the length and spacing of the holes so that the hole length is 24 mm, 28 mm, 32 mm, 36 mm, 40 mm. 44 mm, 46.9 mm, 48.7 mm (through hole), the above model was calculated by ICEPA K software. The maximum temperature of the heat dissipation frame and the maximum temperature distribution of the heat source in the model are shown in Fig. 5.
5 frame maximum temperature and the corresponding relationship between the maximum temperature of the heat source and the length of the hole. It can be found that the maximum temperature of the heat dissipation frame and the maximum temperature of the heat source decrease with the increase of the length of the opening, decrease to a certain minimum value, and then increase until the hole is full. Opened. This is because as the length of the opening increases, the hole area increases, which facilitates gas flow and enhances the heat dissipation performance of the heat sink. However, as the hole length continues to increase, the heat sink width decreases, and the heat dissipation effect of the heat sink is weakened. Raise. It can be obtained from the calculation result that the heat dissipation effect is optimal when the total length of the opening is about 90% of the total length of the frame.
3. 3. 2 The optimal design of the number of holes keeps the total area of ​​the openings and other dimensional factors unchanged. Change the number of holes, the length and the hole spacing, so that the number of holes is 9, 10, 11, 12, 13 14. The above model is recalculated using ICEPAK software. The maximum temperature of the heat dissipation frame and the maximum temperature distribution of the heat source in each model are shown in Fig. 6.
Observation 6 shows that the maximum temperature of the heat dissipation frame and the maximum temperature of the heat source decrease with the increase of the number of openings, but the change is not obvious and tends to be stable. This is because as the number of openings increases, the probability of air flow overheating source increases, which facilitates the flow of heat away from the gas flow, so the maximum temperature of the heat dissipation frame and the maximum temperature of the heat source decrease; however, the total area of ​​the main opening does not change. , so the temperature change is not obvious. At the same time, the number of openings increases, and the processing difficulty increases, which will increase the production cost.
Thermal analysis and optimization design of 4 solar flower-shaped heat dissipation structure
4. 1 Sun flower-shaped heat dissipation structure model Simplified solar flower-shaped radiator is another heat dissipation form widely used in LED lighting fixtures. For a sun flower-shaped radiator, the thermal analysis model is established by ICEPA K software. The original lamp model and size are 7. In order to optimize the geometric parameters of the heat dissipating fins, the radial fins around the heat sink are simplified during the modeling process, and the flat fins in the original structure are replaced by the flat fins of the original heat radiating fins. The simplified fin heat dissipation area is the same as the original structure, which basically ensures that the simplified structure can accurately reflect the heat dissipation effect of the original structure, and at the same time facilitates the parameter optimization of the fin. The numerical analysis model is shown in 8.
The green part of 8 is the area where the heat source is located, and the heating power of the heat source is 1 W. The heat transfer coefficients of the heat sink, the aluminum substrate and the insulating layer are 170 W/mK, 150 W/mK, and 1.8 W/mK, respectively. The constant temperature (40) is used as the boundary condition.
4. 2 Sun flower shape heat dissipation structure analysis Using ICEPAK software, similar to the thermal analysis process of strip radiator, the temperature field distribution of the solar flower heat dissipation structure is calculated, as shown in 9.
Observation 9 shows that the highest temperature of the solar flower-shaped heat sink is distributed near the heat source point, the overall temperature of the aluminum substrate is higher, and the radial fin temperature decreases with the radial distance from the center of the aluminum substrate. The highest temperature rises to 19. It can be considered that the radial fin distribution in this example makes the air flow smooth, the natural convection can be fully performed, and the overall temperature of the lamp meets the relevant industry specifications of not more than 65.
4. 3 heat dissipation performance of the solar flower-shaped heat dissipation structure The traditional LED heat dissipation structure design is often based on the individual experience of the engineer, and it is difficult to ensure that the given design is feasible and optimized. This project introduces the numerical heat transfer analysis technology into the design process of the radiator, establishes the fin parameter optimization model of the solar flower radiator, and uses rational structural optimization technology to optimize the design parameters of the radial fins and carry out different parameters. Parameter fitting provides quantitative recommendations and foundations for the design of power LED lamp radiators.
Optimization model: T o find L if in M ​​in W = N i = 1 i L if in ti S. t. T max 65 (5) Considering that the cost of the heat dissipation structure is directly related to the amount of heat sink material, we have a heat dissipation structure. The total weight is the optimization target. The length of the radial fins and the number of fins are the design parameters. The constraint temperature is 40. The maximum temperature of the lamp does not exceed 65. The purpose is to find the allowable temperature as the power of the lamp increases. The optimum fin length and thickness (minimum heat sink weight) are required, and the influence of the number of fins on the heat dissipation performance of the heat sink is also discussed.
4. 3. 1 fin length parameter optimization The number of fixed fins is 36, the fin thickness is 2.2 mm, and the ambient temperature is 40. Under different heat source powers, optimization design is carried out by parametric modeling and analysis techniques. The maximum temperature does not exceed the minimum fin length for the permissible temperature (65), which is the lightest radiator weight. The total heat source power corresponds to the shortest fin length required for heat dissipation.
0 The corresponding relationship between the total heat source heat source and the fin length required for heat dissipation can be seen from 0. The total chip power is between 12W and 22W, and the required fin length changes are relatively large; as the heat source power increases, The length of the fins required is also increased. It is also found that the length of the fins increases rapidly with the increase of power in the 12W 14W interval. The slope of the curve is larger at 0, and then the required fin length and the total power of the chip exhibit a linear increase relationship. .
In order to facilitate the use of engineering designers, the relationship between the optimal chip length calculated by a large number of chips and the total power of the chip is linearly fitted (Y is the optimal chip length, X is the chip power), and the fitting formula is given as Y= 4. 0333X+ 34. 422
4. 3. 2 The influence of the thickness of the fin on the temperature of the lamp. For the original sunflower-shaped radiator structure shown in Figure 7, when the ambient temperature is 20, the length of the fin and the number of fins are kept unchanged, and the thickness of the fin is changed. Calculate the effect of different fin thickness on the maximum temperature of the heat sink. The calculation results are listed in 2.
This shows that the change in the thickness of the fin has little effect on the operating temperature of the lamp. This is because the increase in the thickness of the fins does not effectively increase the heat dissipation area of ​​the fins, but instead increases the weight of the heat sink and increases the cost.
However, considering that the heat sink fins are formed by extrusion process, there is a certain lower limit on the thickness. According to the extrusion process experience, it is recommended that the fin thickness be as large as possible to ensure the heat sink manufacturing cost is reduced by more than 1 mm.
4. 3. The influence of the number of fins on the temperature of the lamp. Under the condition of an ambient temperature of 20, the original design of the heat sink shown in 8 keeps the fin thickness and the fin length unchanged, and gradually reduces the number of fins. Study its effect on the temperature distribution of the heat sink.
Through the ICEPAK software, the structure of the radiators of 9, 12, 18, and 36 fins is modeled and calculated, and the relationship between the maximum temperature of the heat sink and the number of fins is obtained.
1 The relationship between the maximum temperature of the heat sink and the number of fins can be seen from 1 . As the number of fins increases, the maximum temperature of the lamp gradually decreases. Increasing the number of fins can effectively reduce the temperature of the lamp. However, the number of fins is limited by the processing technology, and it is impossible to increase indefinitely; on the contrary, the arrangement of the fins is too dense, so that the viscosity of the boundary layer is enhanced, the convection is not sufficiently performed, and the heat dissipation effect is deteriorated.
According to the calculation experience, the fin spacing needs to be greater than 4 mm to ensure the smooth progress of natural convection.
5 Conclusion
Based on the design idea of ​​numerical simulation, this research project uses ICEPA K thermal analysis software to establish the thermal performance analysis model of LED lamps. The thermal analysis of the strip structure and the solar flower heat dissipation structure commonly used for the two LED lamps is given. Simulation results of temperature field and flow field of radiator and lamp structure. Based on the basic principle of heat transfer, through the analysis of the temperature field and flow field results of the prototype design, the deficiencies of the prototype design are found out, and the improved design is proposed. It is found that proper opening can effectively improve the heat dissipation effect of the heat sink. Reduce material usage and reduce LED lamp costs. A parametric model of the radiator structure is established for the sun-shaped radiator. The total weight of the heat-dissipation structure is optimized. The maximum temperature of the lamp is constrained. The parameters of the heat-dissipation structure are optimized, and the optimal wing with different thermal power is given. The length of the film was found to have no effect on the maximum temperature of the heat sink. The influence of the number of fins on the temperature field of the LED heat dissipation structure was discussed. Based on the numerical fitting technique, the engineering fitting formula which satisfies the optimal fin length under the highest temperature constraint in the industry is given, which is convenient for engineering design and provides quantitative data support for the development and finalization of new products for power LED lamp radiators. And a reliable scientific basis.
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