Bingjing Wu

Visual investigation on the heat dissipation process of a heat sink by using digital holographic interferometry

We present a method for visually and quantitatively investigating the heat dissipation process of plate-fin heat sinks by using digital holographic interferometry. A series of phase change maps reflecting the temperature distribution and variation trend of the air field surrounding heat sink during the heat dissipation process are numerically reconstructed based on double-exposure holographic interferometry. According to the phase unwrapping algorithm and the derived relationship between temperature and phase change of the detection beam, the full-field temperature distributions are quantitatively obtained with a reasonably high measurement accuracy. And then the impact of heat sinks channel width on the heat dissipation performance in the case of natural convection is analyzed. In addition, a comparison between simulation and experiment results is given to verify the reliability of this method. The experiment results certify the feasibility and validity of the presented method in full-field, dynamical, and quantitative measurement of the air field temperature distribution, which provides a basis for analyzing the heat dissipation performance of plate-fin heat sinks.

Measurement and reconstruction of three-dimensional configurations of specimen with tiny scattering based on digital holographic tomography.

Based on digital holographic tomography, a method for measuring the three-dimensional (3D) configuration of a transparent object with tiny scattering is proposed. By rotating a Michelson interferometer around static specimen, a series of the phase change distributions of reconstructed object beams carrying the structure information of specimen can be obtained at complete angle range, and the rotation of specimen can be avoided. In addition, phase multiplication of the interferometer can be used to improve the measurement sensitivity of specimen with tiny scattering. And then, as an example, the tomographic maps and 3D configurations of an ultrasonic standing wave field are measured according to the filtered-backprojection algorithm. The comparison between experimental and simulation results is provided to certify the feasibility and reliability of the proposed method.

Visual and dynamic measurement of temperature fields by use of digital holographic interferometry

The measurement of temperature field distribution in transparent media is an attracting subject in many research fields. In transparent medium, the temperature change will induce a corresponding refractive index change and thus lead to the phase distribution and variation of the object wavefront passing through the medium. Different from the traditional optical holographic interferometry, the recently developed digital holographic interferometry allows recording the hologram using digitally imaging devices such as CCD, and reconstructing the holographic image by numerically simulating beam diffraction, so it can directly obtain the complex amplitude distribution of the object wavefront in different states by dynamically recording a series of holograms of the object field in different time. Then more detail information can be calculated, such as the refractive index distribution and variation in transparent media. In this paper, we introduce the principles and review some of the applications on dynamically measuring the temperature field distribution by using digital holographic interferometry with specially designed experimental setups, such as the temperature distributions and variations corresponding to Rayleigh-Benard convection, heat conduction process in glass samples, heating process of the oil in container, flame filed, cooling process of heat sink and so on.

Dual wavelength digital holography for improving the measurement accuracy

In dual wavelength digital holography, a synthetic wavelength is obtained by using two lasers with different wavelengths to expand the measurement range of samples’ step heights from nanometers to micrometers. However, its measurement accuracy reduces along with the expansion of measuring range and significant noise is introduced at the same time. For cases where the sample’s height is smaller than the wavelength of illumination light, the measurement accuracy is very important. In this paper, a new approach of dual wavelength digital holography is presented. The synthetic wavelength is smaller than the wavelength of the two different lasers. Higher measurement accuracy can thus be achieved. The analysis and experimental results show the validity of this method.

Real-time visualization and analysis of airflow field by use of digital holography

The measurement and analysis of airflow field is very important in fluid dynamics. For airflow, smoke particles can be added to visually observe the turbulence phenomena by particle tracking technology, but the effect of smoke particles to follow the high speed airflow will reduce the measurement accuracy. In recent years, with the advantage of non-contact, nondestructive, fast and full-field measurement, digital holography has been widely applied in many fields, such as deformation and vibration analysis, particle characterization, refractive index measurement, and so on. In this paper, we present a method to measure the airflow field by use of digital holography. A small wind tunnel model made of acrylic glass is built to control the velocity and direction of airflow. Different shapes of samples such as aircraft wing and cylinder are placed in the wind tunnel model to produce different forms of flow field. With a Mach-Zehnder interferometer setup, a series of digital holograms carrying the information of airflow filed distributions in different states are recorded by CCD camera and corresponding holographic images are numerically reconstructed from the holograms by computer. Then we can conveniently obtain the velocity or pressure information of the airflow deduced from the quantitative phase information of holographic images and visually display the airflow filed and its evolution in the form of a movie. The theory and experiment results show that digital holography is a robust and feasible approach for real-time visualization and analysis of airflow field.