Abstract: The overall performance of a two-dimensional symmetric straight-walled dump diffuser in combustor is investigated experimentally. The test model consists of a straight-walled pre-diffuser with fixed diffuser wall angle(2 = 12°) ,a semicircular head flame tube without holes, two throttles for varying flow split ratio(S)between upper and lower channels surrounding the flame tube, add eight different diffuser shells, The experiments were carried out under the conditions of Main =0.277 and S = l, and for each of the eight shells, the dump gap (D) was changed 4 times. The main purpose of the tests is to study the influence of the different shell shapes and dump gaps on the overall performance of the dump diffusers.The results show that their influence on the loss of total pressure of the diffuser is strong and the maximum loss occurs when the front wall of the shell is located in the pre-diffuser outlet section. However, the change of shell shapes has only small influence on optimal dump gap (for minimum loss of total pressure). In all the test cases, the optimal dump gaps are within D = 40mm and D = 50mm. The experiments also indicate that under certain operating conditions there exist low pressure areas around the head of the flame tube, The external static pressure in these areas can be lower than the pressure in the flame tube. Therefore, any holes in the areas should be placed with great care to avoid reversed flow.